Pole sitter Marquez lost position in the opening laps of the race to drop to fourth when he clipped Pramac Racing's Jorge Martin (25) before the Honda rider barged straight into RNF Racing's Oliveira, who was in second place.
While Martin managed to regain his balance and rejoin the race, Marquez and Oliveira were not so lucky, with the latter visibly in pain after the Honda machine wiped him out while he was taking the turn.
"First of all I'm very sorry to Oli, his team, to his fans at the Portuguese GP. I did a big mistake," Marquez told MotoGP.
"It was not my intention obviously, my intention was not even to overtake Martin at that point. I had a massive lock on the front (tyre), maybe the hard front (tyre) option was not ready.
"After that big lock I released the brakes and when I released the brakes the bike goes inside and I was able to avoid contact with Martin but not with Oli."
The Portuguese crowd looked on in despair as Oliveira missed out on a potential podium on home turf and Marquez apologised to the fans in the grandstand after he returned to the paddock.
While Oliveira was declared fit to race next weekend in Argentina after medical checks showed he suffered only a contusion to his right leg, Marquez suffered a possible fractured hand and will undergo further examinations in Barcelona.
The six-times premier class champion was also handed a double long lap penalty - if he is fit to ride in Argentina.
"I will be penalised in Argentina GP for that mistake, something I completely agree (with). Sometimes these kind of things happen in races and I understand I made a big mistake," he added.
"I have a few injuries but, now, for me it's not important. What's important is Miguel is ok. I will come back when I'm ready."
Honda team manager Alberto Puig said it was 'impossible' for Marquez to avoid the crash.
"His front wheel locked and after he released the brake, the bike just exits like a rocket. He was using the hard tyre, so probably the tyre didn't have enough temperature," Puig said.
"He was not trying to overtake, he was not on the limit... We just want to say from Honda we are really sorry for this incident and we hope Oliveira is good."
