Roy Oswalt probably isn't going to throw a pitch in the majors again until the end of June, at the earliest. It's possible he isn't going to be ready until early July. What kind of financial commitment would the Rangers have to lay out for three to four months of pitching? From Scott Miller:

Oswalt and the Rangers have agreed to a deal that will pay the veteran right-hander $5 million for the rest of this year, with $1 million available to him in incentives, according to sources with knowledge of the deal.

If Oswalt were willing to take a deal from the highest bidder this offseason, he probably would have received at least two years. If he didn't have a troublesome back, he probably would have had three-, four-, or five-year offers. In the end, he's still getting paid a decent amount of coin, he gets to pitch for a team that satisfies his offseason demands, and he picked up a couple months of extra vacation. There are worse fates.