Carl D. Offer, a former Harvard graduate student and teaching fellow in Mathematics, was sentenced to four months in jail yesterday for assaulting Dean Watson during the occupation of University Hall last April.

Middlesex County Superior Court Justice Robert Sullivan handed down the sentence yesterday morning, one day after a jury had upheld a lower court finding that Offner was guilty of assault and battery for having helped eject Watson from the building.

Offner-who denied at the trial that the touched Watson during the incident on April 9-will begin serving the sentence in the Middlesex County House of Correction in Billerica next Thursday.

One Week Postponement

Justice Sullivan denied a motion by Offner's lawyer, Daniel Klubock, to stay the execution of the sentence pending an appeal to the Massachusetts Supreme Judicial Court, but Sullivan did grant a one-week postponement to give Offner time to put his affairs in order.

Klubock declined to say last night exactly what further steps he plans to take in the case. To appeal at this point Klubock would have to prove that an error during the court proceedings had denied Offner a fair trial.

Offner was convicted Wednesday after a two-day trial at which Watson testified that Offner had held him by the arm-but had not injured him-while forcibly evicting him from the Harvard building.

His conviction came only six days after another Middlesex County jury had acquitted 19 persons accused of trespassing in University Hall during last April's sit-in.