According to Baseball America, Brian Bixler, Angel Sanchez, and Travis Buck - Astros sometimes-minor-leaguers - have elected free agency:
Bixler got 96 plate appearances in 36 games, "hitting" .193/.253/.330 with 2HR, 36K:7BB. At OKC, he hit .269/.346/.361.
Sanchez went from being the Astros' Opening Day shortstop in 2011 to a minor-league free agent at the end of 2012. In 102 games for OKC in 2012, he hit .320/.390/.407, with 25K:40BB - the best minor-league season of his career.
In 33 games (81 PAs) for Houston in 2012, Buck hit .216/.284/.311, with six extra-base hits. In 22 games for OKC this season, he hit .359/.414/.469.