I'm looking at buying my first bike, which I'll be using to train for and race in tris. I found an OCR3 for a really affordable price of Craigslist. But the guy selling it says he's 5'8", which I am too, and his stand-over barely clears the top tube. Is this bike going to be too big for me and I should pass? Or if I take it to a shop they will be able to adjust the bike to make it workable?

Go try it out. Unless he gave you his inseam and the exact ground-to-top tube measure, you can't really compare how your fit will be to how his is.

Also, if you clear the top tube, that's satisfactory. The 1"-1.5" guideline is only vaguely representative of one aspect of a good fit. Hell, just wheels and tires can affect TT height.