Bareback is particularly risky because with all that vigorous thrusting you're both creating tiny tears in the skin which nasties will joyfully jump out of and set up home in. But most of them need *direct* contact.
RO and OWO are potential transmission routes for getting stds orally, but it's really unlikely. For a live nasty to jump across both these two, AND bridge across DFK... it's really not happening.
The worst realistic scenario is getting a bacterial infection from oral-genital contact either way. It's not at all likely, and antibiotics will sort you out if you run out of luck. If you're dumb around sores (don't go near girls with cold sores, and have a good look before you stick it in...) you could get cold sores/herpes by the same route. In all of these scenarios, you're defiantly never getting AIDS or anything.