The Washington Post Co. yesterday agreed to sell radio station WTOP to the Outlet Co., of Providence, R.I., for $6.675 million in cash and notes.

Outlet, a rapidly growing retail and communications company that owns the local Philipsborn chain of women's specialty shops, said it intends to continue the radio station's all-news fomat for at least two years.

If approved by the Federal Communications Commission, the sale will remove the Post Co. from the radio broadcasting business here. But a spokesman for the firm, vice president for finance Martin Cohen, said the Washington company plans to retain its ownership of WTOP-TV (Channel 9), uncless required to divest under a court decision now on appeal before the Supreme Court.

Under terms of the agreement. Outlet Co. will retain use of the WTOP call letters and the Post Co. will have to select new call letters for its television property. Cohen said the Post Co.'s investment in WTOP-AM currently is slightly under $1 million.

Outlet already owns television stations in Providence, Columbus, Orlando and San Antonio and radio stations in Providence and Orlando. Earlier this year, Outlet agreed to sell WNYS-TV in Syracuse, N.Y., for $11 million to a group that includes former Washington Post Co. president Larry H. Israel.

The announcement that Outlet will retain WTOP's all-news format becomes a condition of the sale and was a relief to employees of the radio station, who have been subjected to rumors in recent months that because of lower ratings, a potential new owner of WTOP would change formats.

There was no secret that the Post Co. was seeking to sell the radio station. In 1971, the Washington company donated WTOP-FM to Howard University, which now operates the station as WHUR-FM. In 1976, the company also sold WCKY radio in Cincinnati as part of a program to "focus broadcasting activities in the television field," according to yesterday's announcement.

Orrin McDaniels, general manager of WTOP, said yesterday he feels "very confident" about the future under Outlet management. "The fact that they are going to continue the all-news format is very important," he added.

Morrie Alter, a news anchorman for the CBA affiliate, said "everybody is catatonic . . . the one thing they wanted to hear most was that the station will stay within the all-news business."

Added editor namie McIntyre: "I think it's a pretty positive thing, though it is always kind of scary when your station is sold."

Bruce G. Sundlun, president of Outlet, said in a telephone interview that his firm plans to "enhance WTOP's competent news staff and fine reputation" with some new equipment, at a new location in the District. Currently, WTOP shares Broadcast House in Northwest Washington with WTOP-TV.

He called the proposed acquisition "a major step in strengthening our broadcasting group" and confirmed reports that WTOP will provide radio reporting from Washington for other Outlet stations.

Sundlun has homes in Providence and Washington and a farm in Virgina. he is a former member of the D.C. law firm of Sundlun, Shear and Singer; chairman of Executives Jet Aviation; and a director of Communications Satellite Corp. He previously was an assistant U.S attorney here.

"WTOP runs the best all-news operation in Washington and I don't see any sense of changing from a winning operation," he added.

The decision by Outlet to keep WTOP in the all-news business pits that station against WRC radio as the sole all-news stations in the market. WAVA in Northern Virginia, being sold to a group headed by Alexander W. Sheftell, is planning to discontinue the all-news broadcasts it pioneered in this area, starting in 1967.

Officers of WRC, wich is owned by the National Broadcasting Co., earlier decided to stick with a news format and to boost the station's reporting staff after the NBC radio network discontinued a short-lived national all-news operation.

Broadcast industry insiders have said that WRC management expected WTOP to discontinue its news format if the property was sold but WRC officials were not available for comment last night.

WMALAM, recently purchased by the American Broadcasting Co. from the Washington Star, long has ben the area's dominant radio station. In various ratings periods this year, WTOP has been as low as tenth while WRC has gained in audience share and passed WTOP on occasion.

A Media Trends report for October listed WTOP as sixth in the local market with 339,600 listners, acccording to McDaniels, ABC paid $16 million to buy both WMALAM and WMAL-FM, in the biggest radio deal on record. Broadcasting magazine said the Post co. was seeking $8 million for WTOP.

WTOP is considered a prime property because of its 50,000-kilowatt broadcast power, the top level permitted by the FCC. It was founded in 1926 and broadcasts at 1500 on the AM dial.

In 1949, the Post Co. and Columbia Broadcasting System purchased WTOP; in 1954, the Post bought out the CBS minority interest.

Cohen, of the Post Co. said Outlet would make a down payment of $2 million and pay the balance over five years. The agreement includes both cash and notes.

In addition to Philipsborn, Outlet owns 166 menswear, women's and department stores. The Post Co. owns Newsweek magazine, The Washington Post newspaper, the Trenton, N.J., Times, and TV stations in Miami, Hartford and Jacksonville.

Media broker for the transaction was Howard E. Stark, of New York, according to yesterday's announcement.