Hey Forever,
The sensible thing to do is nothing for a few months post surgery. After that opinions differ. Seems the biggest concern is a hernia, but I'm not sure there's data that suggests a correlation between exercising with an ostomy and an increased occurance of hernias or not. Seem some folks are just more suseptible to them than others without considering exercise. And obviously it depends on the skill of the surgeon when he placed your stoma.
I haven't had any hernia type issues with mine while doing situps, but my barrier isn't a big fan of me flexing longitudinally across it without it reminding me by leaking. So my plan is to move from situps and such to Pilates for my abs. If interested you might try talking to "Pilatesgirl" on here, as she's an instructor. She pops in on occasion, so you'd probably do best to PM her if interested.
One other thing.......from what I've read on here it seems a hernia in the stoma region is not a trivial thing, or something to be taken lightly. You either learn to live with it or have it surgically corrected, with no guarantees. So best to avoid the whole mess if possible. Just my 2 cents.
regards,
bob