A320 First Officer reported failure to notice temperature exceeded parameters for a FLEX takeoff resulted in an ECAM alert and a rejected takeoff.

Narrative

On preparation for takeoff from ZZZ runway XXL we setup to perform a flex takeoff. During setup I failed to identify that the outside air temperature was within 5 degrees of our set flex temp. OAT was 34 and flex temp was 36. During the takeoff maneuver after setting the thrust levers in the MCT (Maximum Continuous Thrust) detention per takeoff procedure I noticed asymmetric thrust requiring me to input a lot of right rudder to maintain centerline. The captain quickly took controls and performed a reject at around approximately 65 knots. I called tower and informed them we were rejecting and we exited the runway. After exit the brakes were hot. The captain coordinated with Maintenance Control and we called ops to taxi back to the gate. Later we discussed with a jumpseater who had been on board and he asked what the ECAM ENG THRUST LEVERS NOT SET" meant and said he had seen that ecam just prior to our reject. However due to maintaining aircraft control my crosscheck was mostly outside and at the engines indications and I never saw the ecam before the reject. If I had seen it we could have set TOGA and continued the takeoff. Moving forward I will make sure to pay closer attention to OAT and follow the bulletin posted for this situation."