Portland (5-1-7) beat United for the first time in four meetings since the club joined MLS in 2011, extending its club-record unbeaten streak to 11 games.
Dwayne DeRosario scored the only goal for United (1-8-1), which lost a franchise-record seventh straight contest by a combined margin of 18-4.
Henry opened the scoring in the 29th minute, and Olave scored in the 36th to help New York (2-3-2) win for the first time in its last five tries at RFK Stadium dating to April 2011.
Ben Speas scored 13 minutes into the second half to lead the Columbus Crew to a 2-1 victory over D.C. United on Saturday
United (1-1-0) stretched their home unbeaten run to 17 matches (13-0-4) with the win. Their last home loss came in last year's season opener against Kansas City.
In a rematch of the 2012 MLS Eastern Conference championship, the Dynamo took a 1-0 lead on an own goal by United's James Riley in the 80th minute.
This past February, D.C. United coach Ben Olsen spoke of his club's arduous quest to bring in a true goal-scorer, then looked to prized signing Hamdi Salihi and declared, "We think we've found him."
MLS on Wednesday postponed Game 2 of the Eastern Conference semifinal between D.C. United and the New York Red Bulls, determining continuing snow brought on by a nor'easter had made the field unplayable.
Considering the blend of player embraces and fan exultation that engulfed RFK Stadium last weekend, one might get the sense D.C. United's playoff-clinching win over the Columbus Crew marked a mission accomplished, as far as the regular season goes.