Mobil Corp., the nation's second-largest oil company, announced this week it will locate 500 to 600 of its employes in a 12-story office building adjacent to the Fair Oaks shopping mall, a move warmly welcomed by developers trying to market the fast-growing area.

Mobil has signed a five-year lease for the entire One Fair Oaks building, a 215,000-square-foot office tower completed in June by Evans Co., a Northern Virginia development firm. Occupancy is slated for next June.

Officials from Mobil and Evans declined to disclose the terms of the agreement.

Mobil intends to use the building on an interim basis as it prepares to relocate its world headquarters from New York to the Merrifield section of Fairfax County sometime in the next three years. Mobil already has 1,100 employes from its marketing and domestic refining operations at its 130-acre Merrifield complex, located near Rte. 50 and the Capital Beltway.

A portion of those employes will move into the Fair Oaks building as the Merrifield structure undergoes a major facelift in preparation for the oil giant's move from New York. Mobil announced the headquarters' relocation last April, much to the delight of Fairfax officials and local developers, who viewed the move as a major boost in their attempts to market the county as a place in which other Fortune 500 companies can do business.

Aside from the renovation of the existing building at Merrifield, Mobil also intends to construct a new structure that will house about 1,600 employes. Mobil entered the Fairfax market as one of the county's largest landowners when it relocated its marketing and domestic refining divisions from New York in 1980.

Mobil is not the first major company to chose the Fair Oaks area. TRW Federal Systems Group, a defense contractor; Mohasco Corp., a furniture and carpet manufacturer, and the Northern Virginia offices of Aetna Life and Casualty have all decided to move to the massive Fair Lakes office and residential development just west of the shopping mall.

Mobil officials said they are undecided if they will stay in the Fair Oaks site beyond the terms of their five-year lease.

"We want to keep our options open," said John Lord, a Mobil spokesman.

Fair Oaks developers, bent on adding credibility to their building sites, would like to believe that Mobil is moving to Fair Oaks because of its location. Mobil "felt the I-66 corridor was ideal," said Brian F. Tucker, Evans' vice president for leasing.

Mobil may have liked the I-66 corridor, but that was not the driving force in locating at Fair Oaks.

"It's quite simple," Mobil's Lord said. "One Fair Oaks is one of the few buildings in the area that had the square footage we needed at the right time. I don't want to prejudice other areas {because} we just went shopping for an available building in the general {Fairfax County} area."

For developers who are sinking millions of dollars into the booming Fair Oaks area, it doesn't matter why Mobil chose their congested, crossroads location. It only counts that they are coming.

"The Mobil deal will strike up a lot of activity here," said Tucker, whose firm is also looking to develop other Fair Oaks locations.

Michael Windsor, a regional partner with Property Corp. of America, another key force in the changing face of the Fair Oaks area, agreed.

"It will have a tremendous impact," said Windsor, whose firm is involved with four projects in the Fair Oaks area.