OXON HILL MANOR, where Franklin D. Roosevelt and Winston Churchill, sitting on the terrace, agreed on the invasion of North Africa, will be this year's Decorator's Show House. The 48-room mansion was then owned by Sumner Welles, Roosevelt's undersecretary of state. In 1972, the estate was a prime contender as a residence for the vice president.

The mansion at 6701 Oxon Hill Rd. in Oxon Hill, Md., will be open to the public Sept. 24 - Oct. 22. The women's committee of the National Symphony Orchestra is sponsor of the benefit for the symphony, according to Marcelle Cahill, chairman of the Show House. The black tie invitational preview party will be Sept. 22, under the chairmanship of Muriel Threlfall. Several dozen of the area's top professional decorators will show their designs in the Neo-Georgian mansion.

The house, now owned by the Maryland Park and Planning Commission, sits on 55 acres overlooking the Potomac. During Welles' ownership, the estate included 250 acres. The gardening bill alone was said to be $45,000 a year.

The estate originally belonged to the Addisons, an early Maryland family. The original manor house burned in 1895. According to legend, John Hanson, who preceded George Washington as president of the United States under the Articles of Confederation, is buried on the property.

The present house was built in 1927 by Welles. The Maryland Park and Planning Commission eventually plans to spend $300,000 to turn the building into a Prince George's County cultural center. Some $30,000 worth of roof and plaster repairs will be made before the Show House.

The Chinese rice wallpaper was donated by Roosevelt who apprently appreciated his stay there.

More recently, the house was owned by Fred Maloof, an antique dealer and collector, who amassed a huge store of artwork. At one time, he had an Egyptian mummy displayed in the bed of the master suite. The servants dining room at that time seated 20.

According to a 1977 story by Eugene Meyer in The Washington Post, Maloof, in his 20-year tenure, allowed some hundred people to live in the estate including a Japanese portrait painter and Hungarian refugees. On Sundays, the collection was open to the public. After Maloof's death, in 1972, there was a long controversy over the disposition of his hordes of antiques and paintings. CAPTION: Picture, Oxon Hill Manor, overlooking the Potomac. By Gerald Martineau-The Washington Post