United moved within one point of the second-place MetroStars in the Eastern Conference. It was the third win in four games for United.
The teams meet again Sunday in Washington.
United goalkeeper Nick Rimando finished 50 minutes short of the MLS record mark for consecutive minutes when he was forced to leave with a left knee injury in the 78th minute after making a lunging throw from his own penalty box.
Rimando's exit ended his streak at 6,016 minutes dating to July 4, 2001, just behind the record set by Peter Vermes.
Olsen scored in the 12th minute. Midfielder Earnie Stewart delivered a perfect pass to defender Mike Petke into the box. Petke's header hit the crossbar and Olsen pounced on the rebound.