The most games the Thunder can win is 72.
The most the Blazers can win is 79.
So if Portland wins at their current pace (which is not likely), they'd win 59 wins.
For the Thunder to pass them, they'd have to go 57-12, or 82% winning %. Over an 82 game season, that would be 67. Not unreasonable, but difficult.
If Portland repeats last years pace, 54 wins, the Thunder would have to go 52-17, 75% winning %. Over a 61 game wins per 82 games.
Ideally, if Westbrook and Durant are back by December 1st, it's really just 2 or 3 more losses (Utah and Golden State). But the Thunder have the opposite problem Portland does. they've played 8 road games, so they're going to get a lot of them coming up.
If OKC is 4-12 by the time W and D come back, to win 50 games they'll need to go 46-18 (just a hair under 72%). Over 82 games that's 59 games. Again, not unreasonable.
If they both miss another 3-4 weeks (returning a week before Christmas), these #'s and figures obviously change.