Photo by Markus Thorsen

The great thing about attending a festival in a truly egalitarian place is the notable lack of bullshit. Oslo's Øyafestivalen specialises in an ambience of easygoing efficiency. Despite selling out in advance every year since 2002 and having a relatively compact layout, Øya remains refreshingly roomy. The main stages are situated at the bottom of grassy slopes, creating an amphitheatre effect with near-perfect sound. The crowds are well-mannered and like to have fun without losing the run of themselves. There are no unconscious bodies to step over, no glares of judgement from random passers-by. According to the festival organisers, there hasn't been a single arrest in the event's 16-year history. You'd be hard-pressed just to spot someone relieving themselves in public.

As an outsider, I was curious to know if this could be related to Janteloven, a set of principles made famous by Danish-Norwegian writer Aksel Sandemose as a way of characterising the traditional Scandinavian mindset. (Rule number one: Don't think you're anything special.) Some feel an element of cautious humility will always be prevalent here, others dismiss it as an outdated concept. Posters declaring 'Fuck Janteloven!' and the rise of the rosabloggere ('pink bloggers' writing about their appearance and daily lives) suggest that attitudes are changing.

Either way, selfie sticks are banned from Øya. Crowd surfing is prohibited too. It's also frowned upon to mention how expensive things seem. (A four-day ticket costs £195, while having beer with your fish and chips will come to about £16.) You're meant to see the bigger picture here. Norway is the kind of place where equal living standards are the norm and where everyone under 18 is entitled to the same level of education.

In a festival setting, that translates to a gender-balanced bill that's relatively light on corporate sponsorship and caters to eclectic tastes. That this year's line-up includes Chic, Run The Jewels, Caribou, Songhoy Blues, Laura Marling, Enslaved and Bad Religion gives you a sense of the breadth covered.

Øya also takes environmental issues so seriously that it trains other festivals in how it's done. Anyone willing to pick up enough cigarette butts to fill a beer cup, for example, will be paid 45 NOK (about £3.50) for their effort.

But what about the music? One of the better features of Øya is that the stages are close enough to nip between sets without experiencing any sound bleed. One exception is when Sunn O))) perform.

You can tell it's begun from the eruption that quakes across the festival site. It's 16:30 on a hot Thursday afternoon and, just beyond the sunlight spilling into the Sirkus tent, the stage looks like it's engulfed in flames. The smoke turns from reddish grey to dark black, obscuring a 6ft wall of amps. Occasionally the wafting darkness clears to reveal three robed figures hunched over their instruments. Then frontman Attila emerges from a hole in the ground, his arms outstretched, before disappearing back into the smoke.

'Aghartha' and 'Hunting & Gathering (Cydonia)', from 2009's Monoliths & Dimensions, are torn through with a primal physicality that can't be captured on record. Visually, it's just as mesmerising. Attila conducts the performance as if it were a black mass, his delivery somewhere between a Gregorian chant and an Aboriginal incantation.

The photo pass, I think to myself. They've given me a photo pass for no good reason. This is the perfect opportunity to get that little bit closer. I signal my intention to the security guard. He glances at the pass and nods, presuming my bag must contain camera equipment. "Make it quick," he says. But standing so close to a Sunn O))) performance feels unnatural. The waves of low-end tighten the chest a little too much. Glimpsing the faces of ordinary guys beneath those hoods only hinders the suspension of disbelief.

As if on cue, Attila cocks his head towards me, pointing a crooked finger at the iPhone I'm idiotically trying to take a photo with. Suddenly it feels like whatever spell he's growling out has now incorporates the nuisance before him. It's like that moment in Eyes Wide Shut when the masked party-goers confront an intruder in their midst. There's a tap on the shoulder. It's time to leave.

Later, Attila wears a crown of metal spikes and a full-body reflector suit while wielding a purple laser gun. He guides the show to a close with a cacophony of rasping and maniacal laughter, disappearing into the smoke one final time. His bandmates bow and wave goodbye like malevolent popes. A burly man in his sixties staggers back out towards the light, his eyes flicking from side to side, unsure of what he's just witnessed.

Two hours later, Arthur Russell's Instrumentals begins on the same stage in front of a relatively small gathering. The performance is a new reading of an avant-garde work, first performed 40 years ago, which was inspired by the nature photography of Yuko Nonomura. This incarnation is directed by Peter Gordon, a former collaborator of Russell's who worked closely with him on the original arrangement, and features multi-instrumentalists such as Rhys Chatham and DFA's Gavin Russom.

Perhaps it's the time and setting, perhaps it's the sheet music, perhaps it's just because following Sunn O))) would be difficult for anyone – but the performance feels lacking. The nine-piece ensemble floats along languidly, weaving between each other's contributions in a way that can feel transcendent one moment and pedestrian the next.

Having said that, plenty of those present are entranced – none more so than a dance circle made up of people in their early-twenties. There's something mystifying about watching a guy in short-shorts and a baseball cap doing high-kicks to Arthur Russell compositions. Is it drugs? Are they part of some religious organisation? I'm beckoned into the circle to find out.

One by one, each dancer shimmies towards the centre before breaking into a moment of improvisational madness. The moves grow increasingly funky until I glance across at my companion and mouth the words: "What the hell is happening?"

But then one guy breaks out an imaginary ball and starts passing it around. A line has been crossed. I step out of the circle, slightly alarmed at how enthusiastically this sphere is being fired about, but confident that I have my answer. They're on imaginary drugs.

"Is everybody happy? Is anyone scared?" asks Jenny Hval as she opens Friday's billing on the Vindfruen stage. "I am." But what follows shows no sign of trepidation. Hval's stage presence is so unapologetically idiosyncratic that, in bringing recent album Apocalypse, Girl to life, the Norwegian delivers arguably the most memorable performance of the festival.

She does so with the help of the Apocalypse Girls, who are more of a theatre troupe than backup singers. They are barefoot; dressed in jumpsuits and wigs. They smear each other with red paint, blow balloons while riding an exercise bike and wrap Hval in toilet paper. That may sound like a sixth-form art project but – until Hval's budget can compete with the pageantry of Bjork or the Flaming Lips – it works well.

The exuberance and conviction behind the performance quickly pulls you into Hval's vision. Songs such as 'Kingsize', 'Heaven' and 'Why This?' flit between streams of abstract imagery and confessional storytelling, touching on themes of power and desire. As the portishead-like beat driving 'That Battle Is Over' sputters out and signals the end, Hval crumples into the arms of the Apocalypse Girls, repeating in a whisper: "Sleep tight… forever."

Another highlight is the hypnotic light show animating Flying Lotus's bass-heavy set. Steven Ellison stands between two projection screens the size of the stage, wearing a pair of illuminated goggles that gives his silhouette a spectral quality. The 3D visuals swirl and pulsate around him, the low-end enough to rattle sternums even at the back of the tent. Towards the end, he steps out from the screens to perform as his rapping alter-ego, Captain Murphy, giving a shoutout to the late Sean Price and thanking the crowd with a giddy enthusiasm. Then a pair of socks are thrown on stage. "Uh, yo, where the drugs at?" he asks. This prompts a brief hail of unidentifiable objects. "Okay, that's enough!" he shouts, giggling his way into the darkness.

Things run less smoothly at Holly Herndon's set. It begins with streams of typed sentences, materialising letter by letter on an overhead screen, and the sound of a helicopter about to take off. The messages inform us that this is all being recorded and that normally this part of the show is spent browsing the internet. Given the distance that communicating by text creates, this could sound passive aggressive, if it weren't for the fact that we're invited to participate by submitting messages of our own.

As the music starts up, the visuals (which appear to be controlled by a man on a laptop to Herndon's right) switch to warped 3D collages reminiscent of virtual reality in the 1990s. Herndon's vocals begin to rise, processed and manipulated but still full of warmth, and everything seems to flow with hyperconnectivity. It sounds great. The crowd is absorbed. And then something shifts.

The vocals recede, the audio drifts into ambient loops. The streams of text start up again: "Unexpected interlude," it reads. "It seems the sun doesn't want our computers to work properly." Several minutes pass, the live commentary unrelenting. "Almost there... Or are we just making this up?" People begin looking at each other in confusion. Is this part of the show?

We are reminded to get in touch. The phone number provided is corrected, then changes altogether. Are we being tricked into text bombing some unsuspecting person on the other side of the world? The crowd's murmuring gains momentum. If something is broken, then why is there still audio and visuals?

The text races on, anticipating unforgiving critiques. "This is the sound of our careers being broken." "It's your funeral, babe." Is this all pre-programmed? Or does the guy typing these messages just have a blistering wit? When the visuals change to random Facebook browsing, the crowd stirs uneasily, filing out in dribs and drabs. Rumours circulate. Someone who works at the festival suspects Herndon is pissed at something and this is her idea of protest. Is this meta? Are we being trolled?

No, it turns out. Herndon's computer genuinely overheated. Her frustration is palpable, the apologies are sincere – as confirmed later on Twitter. But it keep people guessing long into the evening, as the sun goes down on a festival full of talking points. All of them a welcome change from the norm.