If you have a oneworld-operated (whose plane you are flying) & oneworld marketed (whose code shows up on your ticket) flight, you can credit to any oneworld FFP.
Check your ticket for fare / booking class. Check the FFP you are crediting to -> choose the marketing carrier (oneworld FFP credits based on the marketing carrier) -> see if that fare code credits you anything. In WheretoCredit, choose the marketing carrier -> respective fare class on your ticket.

e.g. A flight operated by AA coded as IB9997 (your ticket says this), booked in "V" class (your ticket should also show this). You want to credit it to AY. You look here: https://www.wheretocredit.com/iberia/v
It says "V" class on Iberia gives 50% on Finnair. So you can credit this.

Sometimes, a very low economy booking class does not earn anything in your program of choice. e.g. "O" class on CX (Cathay Pacific) does not earn anything (0%) on AA FFP. So if you are flying a "O"-coded oneworld flight marketed by CX, it might be worth it to credit it to somewhere else. But this tends to be the exception, so generally you should choose one FFP and stick with it. The choice of FFP depends on what flights you usually fly (e.g. if you often fly AA-marketed flights, choose the AA FFP) & status, redemption value etc. But once you choose it you should use that and only that for oneworld flights so you don't distribute small amounts of miles everywhere.