Enter criteria and click Find Words
17 five letter words begin with ON, and only two of them are words you would use.
C leads the third box at about twenty nine percent, then I, K and L at twelve each. Nothing else appears more than once or twice.
ONION and ONSET are the entries in ordinary use. Everything else is a rare form or a spelling almost nobody would produce unprompted.
Guess ONSET first, since its letters are commoner than ONION's, and treat the rest of the page as a fallback rather than a plan.
Yes, it uses O in the first and fourth boxes and N in the second and last. Only two of its five letters are distinct.
ON is a complete word on its own, so English rarely builds five letter roots on top of it. What survives is mostly obscure.
ONION turns up more in ordinary writing, but ONSET is the better guess because its S, E and T rule out far more when they miss.