The pre-req to getting an exit row on an AC flight at OLCI is to not have a seat assigned to you, as is the case with many Tango fare pax. I assume that T+ pax would select a seat, unless the flight is oversold or something. Otherwise, one can ask at the airport, however sometimes by that point connecting pax, who speak zero English from overseas flights have already been given the exit rows, and will end up having to move once onboard.