OXFORD – A man who survived a rollover crash on the Massachusetts Turnpike – managing to crawl from his vehicle – was struck by a tractor-trailer minutes later. He suffered life-threatening injuries, police said. The man was one of several people injured in a multivehicle crash about 4:30 a.m. Monday in the eastbound lanes. The accidents occurred near the Oxford-Charlton line, about two miles from the Auburn exit. There were heavy delays on the highway. The chain-reaction accident apparently started when a pickup crashed into the median and overturned, landing in the center lane. A 26-year-old Brookline man and his 25-year-old passenger were able to get out of the pickup, according to state police. A car passed the pickup on the right before the tractor-trailer came by and struck one of the men who had crawled from the pickup, according to state police. He was taken by ambulance to UMass Memorial Medical Center – University Campus in Worcester with life-threatening injuries, according to state police. The other man, who had been the passenger, was not struck by the big rig but suffered injuries in the crash, police said. He is also at UMass Memorial. The tractor-trailer then struck the sedan that had just passed the rolled-over pickup, according to state police. The driver of the sedan and his passenger both suffered serious but not life-threatening injuries, and were both taken by ambulance to UMass Memorial. State police said the tractor-trailer came to rest against the median guardrail after striking the pickup and then the sedan. The driver, a 26-year-old Rhode Island man, suffered serious injuries and was flown by medical helicopter to UMass Memorial, according to state police. The crash caused backups on the Mass Pike and on parts of nearby Route 20.