Rep. George M. O'Brien, 69, a seven-term Republican who represented part of the Chicago suburbs and was highly regarded by colleagues on both sides of the aisle, died yesterday at the National Cancer Institute. He had been suffering from prostate cancer.

Although he seldom won great public notice beyond the borders of his district, Rep. O'Brien stood out from his colleagues in 1979 for his vigorous opposition to a pay increase for Congress.

He also won notice for his visit last year to Syria in which he became the first member of Congress known to speak to Syria's president about the American hostages in Lebanon.

His death was announced yesterday on the floor of the House by Rep. Henry Hyde (R-Ill.), who said Rep. O'Brien died about 5:30 p.m.

A Chicago native and a resident of Joliet, Ill., Rep. O'Brien disclosed on May 16 that he had cancer and was ending his bid for reelection to his 4th Congressional District seat.

"He was an honorable and superb public servant," Sen. Paul Simon (D-Ill.), a former House colleague, said in a statement. "He was kind and he was genuine. He had a good sense of where partisanship gives way to cooperation."

Rep. O'Brien, whose votes generally placed him in the ranks of the conservatives, was ranking Republican on the Commerce, Justice, State and Judiciary Subcommittee of the Appropriations Committee.

In 1982, the subcommittee's chairman, Rep. Neal Smith (D-Iowa), described Rep. O'Brien as typifying the sort of conscientious member on whom the detail work of Appropriations depends.

"We were able to get together and work things out. He doesn't relish confrontation; he'd rather be effective," Smith said.

While on Appropriations he supported funding for programs to aid the handicapped, and proved relatively sympathetic to needs of the Legal Services Corp.

In 1979 Rep. O'Brien took the House floor to challenge a measure increasing congressional salaries. He said he spoke for those Americans "who do not have the opportunity to raise their own pay in the middle of their own contracts of employment."

Last summer Rep. O'Brien spoke to Syrian President Hafez Assad about the Americans held hostage in Lebanon. One of the captives, the Rev. Lawrence Martin Jenco, grew up in the district represented by Rep. O'Brien.

What Rep. O'Brien called "the toughest race of my life" came in 1982 when redistricting forced a Republican primary contest in which he defeated a friend, Rep. Edward J. Derwinski.

A Phi Beta Kappa graduate of Northwestern University, Rep. O'Brien served in the Air Force during World War II, reaching the rank of lieutenant colonel. He graduated from Yale Law School after the war.

Survivors include his wife Mary Lou, two daughters and three grandsons.