His facial structure and effeminate smile and mannerisms.
Please keep in mind I'm comparing Oliviera to the baddest fighters on the planet, I am not comparing them to mainstream John Q. Milquetoasts.
In comparison to the baddest fighters who have ever lived, Oliviera is faint of heart, he quits, has expressions on of his feminine face of "worry," and will sit on his ass, dejected, if he gets overwhelmed too quickly (after quitting).
Please realize many authentically tough fighters have never quit, and get up IMMEDIATELY, if they're conscious. Some fighters would never, EVER "sit" after an injury.
That said, Olivera was pretty close to quitting that again to Chandler, but "pulled guard" through an effort of will/training/experience managed to rally and pull out the victory. Credit to them for that.
However, against Khabib, I am 100% confident Oliviera would shit himself and quit after rounds two or three.
He's not better than Khabib at any aspect of the fight game, especially in determination/toughness/will, and he doesn't have the strength or stamina to last five rounds with the Dagestanian.