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.
onto, on to
“Onto” denotes movement (generally actual physical movement) of an object or body towards a position on a surface:
- The cat jumped onto the table.
In any other sense, write “on to” as two separate words:
- Hold on to what you know to be right.
- She’s moving on to a new challenge.
- After the speech, he moved on to address the next topic.
- Hold on to your hats.
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.