Exact seeding will be determined after each team plays its final regular-season game on Friday. The Thunder won the season series 2-1 and would hold the tiebreaker over the Rockets, although there is no real home-court advantage in the NBA's Walt Disney World Resort bubble.
Westbrook, who spent his entire 11-year career with the Thunder, was traded to the Rockets last summer for Paul - who reportedly had a falling out with Harden last season.
Westbrook will miss the Rockets' regular-season finale due to a quad strain. It's not known if that will affect his playoff availability.
The Thunder's win Wednesday also ensured they will send their 2020 first-round draft pick, which was top-20 protected, to the Philadelphia 76ers as a result of the February 2019 Markelle Fultz trade.
Two other Western Conference first-round matchups are also official as of Wednesday night.
The Los Angeles Clippers will face the Dallas Mavericks in Round 1 after clinching the West's second seed with their win over the Denver Nuggets. The Nuggets' loss locked them into the third seed, so they'll face the sixth-seeded Utah Jazz.