The real playoffs, the American and National League best-of-five championship series, began yesterday with victories built on strong pitching for the Dodgers and the Yankees.

Los Angeles extended its post-season winning streak to four, defeating Montreal, 5-1, while New York held on to beat the Oakland A's, 3-1.

At home in Los Angeles, where the Expos have not won since July 1979, the Dodgers had little trouble with their light-hitting foes. Ron Cey, playing in his first game in more than a month, led the Los Angeles attack, doubling home the first run of the game in the second inning, scoring twice, and making several outstanding plays in the field.

Back-to-back home runs by Pedro Guerrero and Mike Scioscia in the eighth completed the Dodger scoring barrage for a 5-0 lead.

Starter Burt Hooton pitched flawlessly, shutting out the Expos for seven and one-third innings. The only Montreal run came in the ninth off reliever Bob Welch.

Oh Boy

Under the lights in New York, Graig Nettles's bases-clearing double in the first inning was all the Yankees needed. Starting pitcher Tommy John and, as usual, relief ace Rich Gossage combined to hold the A's to one run on just six hits.

Oakland got its only run in the fifth on a single by Rob Picciolo, a double by Rickey Henderson and a run-scoring ground out by Duane Murphy.

Gossage came on in the eighth to quell an Oakland uprising and retired the last five batters to pick up the save.