The Revs were looking comfortable for a crucial away win after Shalrie Joseph had headed the visitors ahead midway through the second-half.
But Olsen rescued a point for United thanks to a superb free-kick from Jaime Moreno. With the assist, former Middlesbrough forward Moreno became the first player to earn 100 goals and 100 assists. The Bolivian striker is already the league's all-time leading goal scorer with 122.
"It's part of the game," Moreno said of the accomplishment. "We have got to do a better job finishing. We had so many chances [where] we could have put the game away, but we never stopped fighting. The assist doesn't mean a lot to me, but I guess numbers count in America."
Olsen added: "It's the 90th minute, we're down and everybody's pushing.
"The ball came to the back post, and I was lucky to sneak in there. It's a tie. It ends up being disappointing."