At this late point, maybe even the Red Sox don't want a reluctant Oswalt. They've got Bard and Aceves set for the rotation with Dice-K coming back mid-season bymping one of them (probably Aceves) back to the pen. Having Oswalt from the get-go sounds great to us fans but it may complicate things for the roster. If they sign Oswalt, it can't be for huge money and he can't have a no-trade clause because if Dice-K comes back and pitches well, they may have to trade Oswalt then to someone who can afford him and keep one of Bard or Aceves in the rotation.
With the deal that Aceves just signed, it appears both he and the team believe his role is yet undefined, hence his bonuses being tied to making X number of starts and/or X number of total appearances. If it were just one or the other, it would indicate a strong preference for one particular role. So if Oswalt is signed, or if one of the mlc lottery tickets (Padilla, Cook, Silva) pans out, Aceves is going to the bullpen with no objections.
Should they then find themselves with six starters once Dice-K is back, I doubt they'd hesitate to push Bard back to the bullpen if need be. Either that, or arrange for someone in the rotation to take a mini-vacation on the DL for a week or two rather than trade away a viable veteran arm in the middle of the season. Unless they could use the opportunity to address a more glaring need, I think the last thing they'd want is to trade Oswalt (or whomever) away to alleviate a roster crunch only to have one of the other starters go down a week later.
Odds are, a solution will present itself when the time comes...the old "things have a way of working themselves out" adage. Regardless, I don't think they necessarily have to base any decision now on what they might have to do once Dice-K is back. If they sign Oswalt, it has to be with the intention of him being in the rotation for the length of his contract.