Planning Research Corp. has signed a lease with Tyson-McLean Joint Venture to occupy the Tyson-McLean III building, now under construction adjacent to PRC's headquarters in McLean.

The signing completes informal agreements reached a few months ago between PRC and Tyson-McLean, a joint venture of Hazel-Peterson Cos. and Metropolitan Life Insurance Co. At that time, Tyson-McLean agreed to revise the design and construction of the six-story, 127,162-square-foot building to meet PRC's requirement's.

The new building and PRC's 440,000-square-foot headquarters, also a Tyson-McLean project, "will be linked electronically by four 4-inch conduits and for all practical purposes will operate as a single unit," according to A. S. Damiani, PRC director of corporate facilities.

PRC plans to start moving into the new building next summer.

Other recent developments in commercial realty in the Washington metropolitan area:

*The first building of Oakton Corporate Center at Interstate 66 and Route 123 in Fairfax County has been completed, the Bethesda developer, Baier Properties Inc., has announced. The principal tenants of the 70,000-square-foot, four-story building will include MRJ Inc., which will occupy 42,000 square feet on 2 1/2 floors, and Continental Insurance Corp., which has leased 17,000 square feet.

*VOB Datsun Sales and Service will establish a new facility for its BMW sales and service operation at 1396 Rockville Pike, currently the site of Herson's Honda, with the help of $2.7 million in financing from the Montgomery County Economic and Business Development Corp. The firm will hire an additional 50 employes with proceeds from the loan, EBDC reported.

*Peerless Paper Mills has purchased about four acres of land at Bollman Place in Columbia from Columbia Industrial Development Corp. The land is valued at $340,000.

*A & V Associates of Washington Inc. has leased 1,541 square feet at 1341 G St. NW, and Destination Unlimited has leased 329 square feet at the same building, from K & K Properties Inc.

*McGuire, Woods & Battle has signed a $1.2 million lease for 4,400 square feet at 1025 Jefferson Ct. NW from Crow Washington No. 1.

*Business International Corp. has leased 1,482 square feet at 815 15th St. NW from the International Union of Bricklayers & Allied Craftsmen. The term of the lease is five years.

*Scott Paper Co. has signed a five-year lease for 1,867 square feet at 1726 M St. NW.