Maryland Cup Corp., maker of Sweetheart cups and straws and Eat-It-All cones, earned $3.8 million (37 cents a share) in its first quarter ended Dec. 31, compared with $3.9 million (39 cents) a year earlier.

Sales for the firm, which is the nation's largest manufacturer of paper and plastic food service products and is based in Owings Mills, Md., slipped to $132.7 million from $135.9 million in the same period a year ago.

"What seems to have been the low point of the recession coincided with what is traditionally our slowest quarter," President Sam Shapiro said yesterday. He said the company expects a substantial increase in profitability as economic conditions improve because Maryland Cup has continued to cut costs and improve equipment and facilities.

Maryland Cup split its stock 3-for-2 on Jan. 12.

D.C.-based Syscon Corp., a software and microprocessor enterprise, increased its 1982 earnings 15 percent to $2.8 million ($1.02 a share) from $2.4 million ($1.05), with more shares outstanding.

Revenues increased to $71.4 million from $61.1 million.

In the fourth quarter, Syscon had profits of $934,000 (34 cents) compared with $761,000 (33 cents), while sales grew to $19.2 million from $15.8 million in 1981.

Jose J. Yglesias, president and chief executive, said the annual figures include a 7-cent-a-share loss resulting from an arbitration ruling on a contract and that the company is contesting the ruling.

Monumental Corp., an insurance company in Baltimore, had net earnings in 1982 of $11.5 million ($1.68 a share) compared with $16.1 million ($2.38) in 1981. For the last quarter, earnings were $2.1 million (30 cents) compared with $4.1 million (60 cents).

O'Sullivan Corp. of Winchester, Va., reported earnings of $5.9 million ($2 a share) compared with $5.4 million ($1.81) in 1981, while sales declined to $71.5 million from $73.7 million. Fourth-quarter profits grew to $1.3 million (44 cents) from $1.1 million (37 cents). Sales grew to $18 million from $17.2 million.