played both TCU and Oklahoma State, so as far as RPI is concerned the outcome of that game wouldn't change BYU's position. Because if TCU wins so that TCU's standing goes up (and BYU's loss to TCU becomes less big of deal), then OSU goes down and BYU's tie with OSU is downgraded. And vice-versa if OSU wins.
That won't be true for all conference games because BYU hasn't played every team in the conference, but for BYU will be true for OSU-TCU game.