Primark Corp. of McLean yesterday reported a slight increase in 1985 earnings to $51.2 million ($2.65 per share) on revenue of $1.9 billion, compared with earnings of $49.5 million ($2.63) on revenue of $2 billion in 1984.

The company said it earned $13.5 million (69 cents) on revenue of $531 million in the fourth quarter ended Dec. 31, compared with $11.2 million (59 cents) on revenue of $483.8 million for the same period the year before.

Primark said its earnings for 1985 "are primarily the result of improved performance in the utility and financial services businesses."

Primark is a diversified holding company whose divisions include Michigan Consolidated Gas Co., a natural-gas distribution company; Financial Services, which operates insurance companies, mortgage lending firms and a savings bank; and Hospital Satellite Network, which offers medical education programs to hospitals.

O'Sullivan Corp. of Winchester, Va., yesterday said earnings had decreased slightly in 1985, to $11 million ($1.46 a share) on revenue of $145 million from earnings of $11.3 million ($1.49) on revenue of $130.8 million the year before.

In the fourth quarter ended Dec. 31, the company earned $2.9 million (38 cents) on revenue of $38.4 million, comparable to the $2.8 million (38 cents) on revenue of $32.8 million it earned for the same period of 1984.

The sheet-vinyl, rubber-sole and injection-molded-plastics maker said the slight dip in earnings was due to higher raw materials costs, increased competition from shoe imports and a general escalation in overhead costs for anticipated new businesses.