offline
One word.
off your own bat
Bat, not back.
on-demand
Hyphenated as an adjective, as in an on-demand webcast.
ongoing
One word. But ask yourself: does this word add anything to the sentence? If not, consider removing it.
online
One word.
opt in, opt-in, opt out, opt-out
No hyphen if it’s a verb. Hyphenated if used as a noun or adjective. You can opt out of cookies by using an opt-out mechanism. You can opt in to receive emails if you tick the opt-in box.
oral, verbal
See verbal, oral.