Billy Clarke's 11th-minute goal settled a dour contest at Brisbane Road as the visitors edged a 1-0 win.
The O's produced a lacklustre display and Slade admitted that his side had been disappointing.
He said: "That was not the standard we wanted. We wanted a bit more from the players, a bit more urgency.
"I think the goal was key. He swung his foot at it, I'm not sure whether it was a cross or a shot but it's gone in the top corner.
"He won't score another one of them, that's for sure. That put us on the back foot for 15 or 20 minutes. They haven't really crated too many chances, they've only had two shots on target all game.
"We've huffed and we've puffed for the remainder of the game to be honest, created a few chances but we've got to put one of those away to give us a lift.
"When you get that goal scored against you, after I thought we started okay, it hurts and it took us a little bit of time to recover and get going again. We played some good pockets of football without hurting them but we did get opportunities.
"We've had 17 attacks compared to their eight but we haven't done enough in the final third."