A buddy of mine who is classing up with Alpha Co. is at TBS right now with Mike Co. He said he dropped his flight contract and as a result is going ground, however he cannot compete for a Ground MOS. He said he can, however, submit a preference list to his SPC and if the SPC agrees with it, he might be good to go. But I think the basic theme is that if you drop your Air Contract, it's going to be needs of the Corps and you likely won't get the pick of the litter.
As for DOR at OCS, I wouldn't recommend it. First of all, you have to deal with the further antagonization from the staff, not to mention the fact that they won't send you home just like that. In all likelihood, they'll take their time out-processing you through Admin. I would think that the only way you would have any chance of getting reaccepted into OCS after you DOR is if you DORed due to emergency reasons, i.e. a death in your immediate family or someone in your immediate family becoming very sick. At least, that would make the most sense to me.