The New York Mets, the veritable Cinderella squad that seemed nestled in the fireplace of the National League Eastern Division just five weeks ago, yesterday took the pennant with a 7-2 victory over the Cincinnati Reds in New York.

Behind righthander Tom Seaver, with ninth-inning relief from Tug McGraw, the Mets exploded a 2-2 deadlock with a four-run fifth inning to win the best of five game series, three to two.

Ed Kranepool, one of the original Mets, stroked an opposite-field two-run single and veteran Willie Mays got a one-run Baltimore chopper scratch hit to highlight the Met offense.

On the other coast, the Baltimore Orioles, in desperation, took to the long ball yesterday to trim the Oakland A's, 5-4, before 27,000-plus in the Oakland Coliseum. Their playoff is tied at two games apiece.

Vida Blue started for the A's and coasted to a 4-0 lead while overpowering the Birds through six innings on two hits. His fastball was popping so much that, against a righthanded line-up, rightfielder Reggie Jackson made seven put-outs.

In the seventh inning, though, Oriole catcher Andy' Etchebarren lashed a three-run home run to left to tie the game at 4-4. Against loser and reliever Rollie Fingers, Bobby Grich opened the eighth with another four-sacker to ice it at 5-4. South-paw Grant Jackson got the Baltimore win.