James Michael O'Conner, 19, a Prince George's County volunteer fireman, died yesterday when the fire truck on which he was riding collided with a tractor-trailer on Rte. 301 in Upper Marlboro, officials reported.

He is survived by his 19-year-old wife and a six-day-old son.

O'Conner was the 11th Prince George's fireman to die while on duty. Three other firemen and the driver of the tractor-trailer were injured in the 1:58 a.m. accident. One of the injured firemen, Cecil Thorne, 42, was listed in critical condition yesterday at the University of Maryland Hospital in Baltimore.

O'Conner, who worked full time as a maintenance man for the county school board, was riding on the back of a truck that had responded to a fire call at the South County Public Library in Deale when the accident occurred. Deale is in Anne Arundel County, with which Prince George's has a mutual aid firefighting agreement.

Four minutes after the original call came in, the Prince George's dispatcher was informed that a faulty alarm system had gone off in the library and there was no fire. The dispatcher immediately radioed this information to the truck.

At 1:58 a.m. the firetruck collided with the tractor trailer, officials said. All four firemen were thrown from the truck and O'Conner, who had been a volunteer fireman for 10 months, landed in the path of the tractor-trailer, which was still moving, officials said.

After being freed from beneath the tractor-trailer, O'Conner was taken to Southern Maryland Hospital where he was pronounced dead on arrival.

Howard Back, 22, and Steve Stough, 21 - the driver and only paid fireman on the truck - were taken to Southern Maryland Hospital suffering from leg and facial injuries.

The driver of the tractor trailer, David C. Heuser, 37, from Haverhill, Mass. was taken to Prince George's County General hospital with two broken legs. Police said he was en route to Richmond from Massachusetts carrying a shipment of grain for the Hall & Cole Produce Co. Chelsea, Mass.

Firemen at the Upper Marlboro station stood outside and talked in low tones about the accident yesterday afternoon.

"Everyone's pretty shook up," said Bob Simms, a fulltime D.C. fireman who also volunteers in Prince George's. "He was a nice, easygoing kid. Kind of on the quiet side. But he was a good fireman. He scored 98 on his exam after training school. He knew what he was doing."

The firemen said that O'Conner generally worked between two and four nights a week although he had been seen less in the weeks just before his son was born on June 10.

Heroes Inc. of Greater Washington, which is run by local businessmen, will provide O'Conner's widow, Billie, and son with an initial check for $2,500 and members of the organization will keep in touch with her there after to find out her needs.

O'Conner is the first county fireman to die in the line of duty since Raymond H. Smallwood Sr. suffered a heart attack while on a call in March 1976.

All 11 men killed in the line of duty have been volunteers and none have died fighting a fire, according to department records. O'Conner was the seventh to die in a traffic accident. Two have drowned, one was hit by a falling tree and the 11th was Smallwood.