Baltimore won their American League's East title for the first time in 17 years, when they defeated rivals the Toronto Blue Jays 8-2.
Alejandro De Aza's bases-clearing triple in the seventh inning was the highlight of the Orioles' title-sealing result, with Ubaldo Jimenez pitching five innings of two-hit baseball for the winners.
The Nationals, meanwhile, made it two National League East pennants in three years when they shut out the Atlanta Braves 3-0.
Washington have only topped the division three times in their history, but have dominated in recent seasons.
Starting Nationals pitcher Tanner Roark threw seven shutout innings against the Braves, who hold only faint hopes of earning a NL wildcard spot.
Atlanta sit five-and-a-half games behind a post-season place, with the Milwaukee Brewers - one-and-a-half games behind - the only realistic chance to oust incumbent holders San Francisco Giants and the Pittsburgh Pirates.
The Brewers won 3-2 after 12 innings against the St Louis Cardinals, however the Giants and Pirates also had wins to hold firm.
San Francisco edged the Arizona Diamondbacks 2-1, to move closer to NL West pace-setters the Los Angeles Dodgers.
The Dodgers remain three games clear atop the division, but were humbled 10-4 by the Colorado Rockies.
Pittsburgh closed to within two-and-a-half games of NL Central-leading St Louis courtesy of a 4-0 shutout win against the Boston Red Sox.
American League wildcard holders the Kansas City Royals and Oakland Athletics both lost, allowing the Seattle Mariners to capitalise and move within a game of a post-season place.
The Mariners thrashed AL West champions-in-wait the Los Angeles Angels 13-2, to heap the pressure on the A's and Royals.
Oakland fell 6-3 to rivals the Texas Rangers while Kansas City were beaten 7-5 by the Chicago White Sox.
The Royals also missed the chance to close the gap on AL Central leaders the Detroit Tigers - 4-3 losers to the Minnesota Twins - who held onto their narrow one-and-a-half-game buffer.
The Miami Marlins' hopes of October action slimmed even thinner, after a 9-1 loss to the New York Mets left them seven games behind the NL wildcard positions.
In other results, the Chicago Cubs trounced the Cincinnati Reds 7-0, the Philadelphia Phillies beat the San Diego Padres 5-4, and the Tampa Bay Rays were 3-0 winners over the New York Yankees.