Walter O'Malley, one of baseball's most powerful owners whose 1958 move from Brooklyn with the Dodgers began the westward expansion of baseball and other professional sports, died today at 75. His death was attributed to congestive heart failure but he had been under treatment for cancer for several months.

Mr. O'Malley had been ill for some time and had been hospitalized at the Mayo Clinic in Rochester, Minn., since June 28. He died there early today at Methodist Hospital.

Mr. O'Malley, whose death followed that of his wife, Kay, by four weeks, had had little to do with the running of the club for several years, although he had built the team into one of the richest in sports.

Survivors include his son, Peter, president of the Dodgers.

"Walter O'Malley was as great an executive talent as I have seen or think I am apt to see," said Baseball Commissioner Bowie Kuhn. "While baseball was his medium, his skills would have flourished in any walk of life. He was unfailing in his support of the commissioner's office and a powerful ally for the good of the game.

"His unique ability, charm and wit are not replaceable. He was my personal friend. To his children, Peter and Terry, and his host of grandchildren, we in professional baseball send our heartfelt sympathy."

Mr. O'Malley, a lawyer, became president of the Dodgers in 1950 and moved the franchise from Brooklyn to Los Angeles eight years later, giving the West Coast its first major league baseball team. Other sports - basketball and hockey - followed.

It was a shrewd move for the Dodgers, who have been one of baseball's most successful franchises for years. The team drew an all-time record 3.3 million fans in 1978, breaking its own mark.

Since 1950, the Dodgers have won 10 National League pennants and four World Series.

"I, like so many others in baseball, will be forever grateful to Walter O'Malley for giving us the opportunity to prove we could do a job in the major leagues," said Buzzie Bavasi, president of the California Angels. Bevasi was the Dodgers' general manager for 18 years before leaving the team in 1968 to become president of the San Diego Padres.

"Baseball has lost a great man and I have lost a great friend," said National League President Charles Feeney. "Walter O'Malley over a period of years did more good for professional baseball than any other one man.He will be sorely missed by the sport, all his friends and, in particular, it is a personal loss for me."

Walter Alston, manager of the Dodgers for 24 years until he retired at the end of the 1976 season, said, "Baseball's going to miss a great man.

"He was not only my boss, but my friend," said Alston, reached by phone at his Darrtown, Ohio, home. "He treated me great throughout my career."

Mr. O'Malley was a hardheaded businessman motivated by profit, but he kept ticket prices down to lure the fans into his beautiful $20 million Dodger Stadium... He was content to make most of his money on parking and concessions - restaurants and souvenir stands in addition to the sale of hot dogs, peanuts and the like.

He exercised behind-the-scenes power among baseball's owners. He was said to have verbally bludgeoned Horace Stoneham to move the Giants to San Francisco from New York so the Dodgers would have a regional rival.

Mr. O'Malley was a heavyset man, phlegmatic in appearance. He seemed to screen his emotions behind owlish glasses. He had a quick tongue and was adept at turning aside questions from the press.

He liked his comforts. He was a frequent diner at a gourmet restaurant and smoked a reported 20 expensive cigars a day. CAPTION: Picture, Walter O'Malley