Per John Blake:
No word at this time what corresponding roster move would be made to make room for Oswalt on the 25 man roster, or on the 40 man roster. We've been assuming Mark Hamburger is DFA'd to make room on the 40 man roster for Oswalt, and with Mitch Moreland heading to the 15 day disabled list (we would assume) with a strained hamstring, that would open up a 25 man roster spot.
This would likely mean that Scott Feldman would go to the bullpen and Justin Grimm would stay in the rotation until such time as Derek Holland is ready to return. However, if Moreland went to the disabled list, then Yoshinori Tateyama could be recalled immediately, without waiting for 10 days after he was optioned. That would then allow the Rangers to send Grimm back to the minors, and keep Feldman in the rotation until Holland is ready.
The fact that Grimm isn't starting this weekend increases the likelihood that Tateyama would be brought back up, or that Grimm would end up potentially going to the bullpen if he stayed up.
UPDATE -- Jeff Wilson tweets that Grimm stays in the rotation, Feldman goes to the bullpen:
#Rangers pick Roy Oswalt to start Friday. Colby Lewis, Matt Harrison, Justin Grimm, Yu Darvish will follow. Scott Feldman back to bullpen.— Jeff Wilson (@JeffWilson_FWST) June 20, 2012