After 21 laps of tight paint-swapping overtakes at a windy Assen, where no-one could escape from the lead group, it was Oliveira who spent the most time working hard at the front and he took the position back from Fabio Quartararo at the start of the final lap.
Lucky not to crash when defending his line coming into the final turns after making the pass, the Red Bull KTM Ajo rider led the way over the line to take his second career win after clinching his maiden victory just two races ago at Mugello.
Behind the Portuguese star, a plethora of excellent performances caught the eye as rookie Quartararo put in another impressive ride to finish second after a succession of tough passes on his Honda, with the Estrella Galicia 0,0 rider joined on the podium by championship leader Danny Kent in third.
The Leopard Racing man had at tempted a break on lap nine but when it was not on the cards, the Briton waited to pounce. At the back of the pack at the start of the final lap, Kent then slipstreamed Navarro and blocked hard to hold on to a podium finish.
Kent's third place extends his overall lead again and he is now 57 points clear of nearest rival Enea Bastianini.
Quartararo's rookie team-mate Jorge Navarro, who briefly led with four laps remaining, recovered well after a warm-up crash had left his bike in pieces, and looked at ease in the lead group, finishing a hard-fought fourth.
Fifth went to Sky Racing Team VR 46 rider Romano Fenati, who eased past Gresini's Bastianini in the run to the line, his fellow countryman finishing a strong sixth.
Brad Binder had struggled to stay with the lead group until the frantic nature of the race brought him back into contention, but despite setting a new in-race lap record the Red Bull rider had to settle for seventh.
The lead riders passed t he wild-card back markers with several laps left to run and were 20 seconds clear of the equally manic battle for eighth, which claimed a few casualties along the way in another massive scrap.
The third Red Bull KTM of Karel Hanika led the way over in line for that hotly contested eighth, just ahead of Ongetta-Rivacold's Niccolo Antonelli.
Britain's John McPhee was in the mix after showing improvements in warm-up and despite being hit several times managed to hold on for a top ten finish for the SaxoPrint-RTG Team.
He held off Mapfre Team Mahindra rider Francesco Bagnaia, who took 11th, with Andrea Migno next across the line for the Sky Racing team in 12th.
The remaining points went to Livio Loi (RW Racing GP) in 13th, Hiroki Ono (Leopard Racing) in 14th and Schedl GP Racing's Philipp Oettl in 15th.
Niklas Ajo was very unlucky to miss out on a point, finishing 17th after crossing the line side-saddle on his RBA Racing Team KTM, using his knee to steer p ast the tyres and the wall after an amazing high-side save on the run to the line.
Efren Vazquez was the first to exit the race after just two laps, sliding out of place while following team-mate Kent.
Alexis Masbou retired to pit lane with 17 laps left to run, while Zulfahmi Khairuddin lasted just one lap longer after tumbling through the gravel after contact with Remy Gardner.
On lap 12 both of the Husqvarna Factory Laglisse bikes were taken out by other riders, first Francesco Bagnaia ran into his Mahindra team-mate Juanfran Guevara, who saved himself but took out Isaac Vinales in the process. Guevara later retired from the race.
Moments later at the same corner Niccolo Antonelli's contact with Maria Herrera saw her also having a frustrating end to her day in the gravel.
