Town were silenced at Manchester United but produced an encouraging display against last season’s third-placed Premier League side.
O’Neil admitted a brief lapse in concentration allowed United to take control, stressing the importance of remaining disciplined throughout matches against elite opponents.
“It was our first real test of a bit of adversity. Sunderland equalising would have been a test of that as well, but this was a more extreme version of it,” he told the club website.
“The feeling that we’d been pegged back and it wasn’t really justified, so maybe that just effected the emotion of the group, and we left the gameplan for a little bit.
“When I watched it back, we didn’t leave the gameplan for as long as I thought we did but that 12-minute spell was enough to take the game from us.
“I think for the group, it was a good one for understanding we can compete at the level, but we need to make sure that we’re right on task to give ourselves the best chance to do so.”