This is obviously big. If the Lakers win today, then they've got third place in the Conference and another Pacific Division title as good as won, with just a trip to Sacramento left on their schedule. If the Thunder win, then the Clippers move ahead of their intracity rivals, and take control of their own destiny. The Clippers would still need to win out to keep their advantage, which won't be easy, but it would be in their hands at that point.
The Lakers are 0-2 against the Thunder this season, and Oklahoma City looking impressive in each win. The game matters to the Thunder as well, who are still chasing the Spurs for the best record in the Western Conference and would also like to remain ahead of Miami for what could be an NBA Finals matchup. Not to mention that less than a week before the Playoffs start, the Thunder don't want to be losing to the Lakers and raising questions about 'experience' and the like.