Although they made appropriate transmissions of their position and intentions; the student and instructor pilots aboard a PA28 failed to select the comm system tuned to the CTAF for their arrival airport. While in their takeoff roll from their first touch and go they spotted another aircraft landing opposite direction; rejected their takeoff and rolled off the end of the runway as the other aircraft went around. Only then did they discover their error.

Narrative

15 NM out from OBE; we began our before landing check and commenced our descent. We made a 10 NM out call; a 5 NM out call and also reported over the field for Runway 32. Once in the pattern; we made regular downwind; base; and final calls. On short final we noticed; for the first time; that we had company traffic in the pattern. It looked as though the traffic just took off. We landed; and I expected to slow; to give our upwind traffic some space. A few seconds later; as the rollout continued; we noticed the aircraft getting bigger in size. We aborted our takeoff with 600 FT remaining; and rolled slightly into the grass past the departure end of the runway. The aircraft we saw was on final approach for Runway 14! He went around. We taxied over to the hold short position of Runway 14 and I keyed my mike forcefully to try and make contact with the 'non-communicating' aircraft. We then noticed our 'transmit' signal flash on our Comm 2; which was tuned in to the FSS frequency 122.6; and NOT the CTAF; 123.00. We switched comms; to hear: 'Cherokee at Okechobee; are you on frequency?' We apologized and replied that we hadn't switched frequencies. The remainder of the lesson was uneventful.