Yes you do - the option comes up after you have paid - but they are requests rather than firm reservations for all sort of complex reasons.
But only 60% of seats are availabe for pre selection so if this limit has been reached the option won't be there so you'll have to wait for OLCI to open.
The accepted advice is to look at the seat ratings on here,
http://www.seatexpert.com and
http://www.seatguru.com and choose the best seats you can now, then check again at OLCI to see if any better ones have opened up.