Threlfall poked home Andy Fleming's right-wing corner from close range two minutes into added time to give the home side a deserved point.
Oxford took the lead in the 13th minute when David Connolly received the ball in the box and turned sharply before drilling a shot that took a sharp defection off Morecambe skipper Mark Hughes which diverted the ball past home goalkeeper Barry Roche.
Oxford went close to a second three minutes later when skipper Johnny Mullins was left free at the far post but saw his volley superbly saved by Roche.
Sampson did much better in the 36th minute when he headed a Kevin Ellison left-wing cross inches wide.
Alfie Potter drilled a shot inches wide of the right-hand post after good work from James Constable as Oxford looked threatening on the break.
Morecambe started the second half brightly with Threlfall seeing an early shot blocked before producing a chance for Fleming but the midfielder fired well over from the edge of the area.
Ryan Clarke was forced into a fine save from Tony Diagne in the 90th minute but could do nothing with the resulting corner which found its way into the Oxford goal.
- Sports & Recreation
- David Connolly
- Andy Fleming