I've been able to hold a one way on AA for 2 weeks just using their web site - no need to call for he first flight.
I don't think the airlines have one way awards, although you can upgrade one way. If you book a one way award ticket they will charge you all the miles then - and now they are all charging fees to reticket award travel - in the past there used to be no fee if you kept the same itinerary, only if you changed routing.
