I'm not sure about AA coded O fare on JL metal, but I think it also won't have advanced seat assignment given it's one class below N.
Can confirm no advance seat selection on AA O fare on JL metal either as AA codeshare or sold as the JL flight.
For the OP, personally I wouldn't be concerned as much with advance seat selection on JAL as I would with another airline. JAL usually blocks a larger number of seats until OLCI or even for airport control (take a look at your specific flights in something like ExpertFlyer). Also, the lower fare buckets that do have advance seat selection are often blocked from selecting some open seats until OLCI as well. Start checking in right at T-72 and you'll probably get a decent seat.