The opening scene in The Odd Couple, of the boys in their regular Friday night poker game, is one of the funniest card sessions ever held on a stage.

If you are worried that there is nothing Neil Simon, the author, or Mike Nichols, his director, can think of to top that scene, relax. The main business of the new comedy, which opened last night at the Plymouth Theater, has scarcely begun, and Mr. Simon, Mr. Nichols and their excellent cast, headed by Art Carney and Walter Matthau, have scores of unexpected ways prepared to keep you smiling, chuckling and guffawing.

Mr. Simon has hit upon an idea that could occur to any playwright. His odd couple are two men, one divorced and living in dejected and disheveled splendor in a eight-room apartment and the other about to be divorced and taken in as a roommate.

One could predict the course of this odd union from its formation in misery and compassion through its disagreements to its ultimate rupture. Mr. Simon's way of writing comedy is not to reach for gimmicks of plot; he probably doesn't mind your knowing the bare outline of his idea.

His skill-and it is not only great but constantly growing-lies in his gift for the deliciously surprising line and attitude. His instinct for incongruity is faultless. It nearly always operates on a basis of character.

Begin with that poker game. Mr. Matthau, the slovenly host, is off stage in the kitchen fixing a snack while Nathaniel Frey, John Fiedler, Sidney Armus and Paul Dooley are sitting around the table on a hot summer night, sweating and grousing at the luck of the cards. The burly Mr. Frey is shuffling awkwardly, "for accuracy, not speed," and the querulous Mr. Fiedler, the big winner, talks of quitting early.

The cards are dealt. Mr. Matthau walks in with a tray of beer and white and brown sandwiches. They're brown in his scheme of housekeeping because they're either new cheese or very old meat. As he opens the beer cans, sending sprays of lager over his guests (surely a Nichols touch), the dealer inquires whether he intends to look at his cards. "What for," Mr. Matthau, the big loser, grumbles, "I'm gonna bluff anyhow."

The sixth member of the Friday night regulars, Mr. Carney, is missing. Evidently he has been away from his known haunts for twenty-four hours and a phone call from his wife informs his friends that she hopes he never turns up. Since they know that he is a man who takes such blows seriously, they fear that he will do something violent to himself.

With Mr. Carney's arrival as Felix, the discarded husband, the principal action begins. Mr. Carney is truly bereaved, a man of sorrows. His eyes are stricken, his lips quiver, his shoulders sag. Even poker gives way before his desolation. The players are too concerned about possible moves by Felix toward self-destruction. When at last they go home, they depart softly and gravely like chaps leaving a sick room.

Mr. Matthau as Oscar, the host, consoles Felix, massaging away the spasms in his neck and enduring the moose calls with which the unfortunate fellow clears ears beset by allergies. Nothing much happens during the rest of the act except that these two inevitably blunder into a domestic alliance, but there is scarcely a moment that is not hilarious.

The unflagging comedy in the remainder of the play depends on the fundamental switch-of the odd couple. Felix is a compulsive house keeper, bent on cleaning, purifying the air and cooking. When the gang assembles for its poker game, Felix has special treats ready for snacks.

Mr. Carney handles the housewifely duties with a nice, delicate, yet manly verve. But he is strict. When he serves a drink to Mr. Frey, he wants to know where the coaster is. The answer-and this is Mr. Simon, the marksman at firing droll lines-is, "I think I bet it."

Mr. Matthau for his part is wonderfully comic as a man who finds his companion's fussy habits increasingly irksome. He walks about with a bearish crouch that grows more belligerent as his domestic situation becomes both familiar and oppressive. There is a marvelous scene in which he and Mr. Carney circle each other in mutual distaste-Mr. Matthau looking like an aroused animal about to spring and Mr. Carney resem-bling a paper tiger suddenly turned neurotic and dangerous.

To vary the humors of the domestic differences, Mr. Simon brings on two English sisters named Pigeon-yes, Pigeon, Gwendolyn and Cecily-for a date with Oscar and Felix. The girls induce more laughter than their names promise. Carole Shelley and Monica Evans are a delight as the veddy British and dumb Pigeons.

Mr. Nichols's comic invention, like Mr. Simon's, shines through this production and the comfortable Riverside Drive apartment invoked by Oliver Smith's set. Just a sample: Mr. Carney left alone with the Pigeons is as nervous as a lad on his first date. When one of the girls takes out a cigarette, he hastens to her with his lighter and comes away with the cigarette clamped in its mechanism.

The Odd Couple has it made. Women are bound to adore the sight of a man carrying on like a little homemaker. Men are sure to snicker at a male in domestic bondage to a man. Kids will love it because it's funny. Homosexuals will enjoy it-for obvious reasons. Doesn't that take care of everyone?