Striker Jarvis has only made three starts this season but was called into action with 10 minutes remaining and a draw looking inevitable, but the former Norwich player had other ideas.
It was deep into five minutes of injury-time when he met a cross from fellow substitute Jason Crowe to send a sublime header past Peterborough goalkeeper Joe Lewis to seal the points for the home side.
Orient, with on-loan defender Adam Barrett making his 500th League start, had gone ahead after 67 minutes when their former defender Gaby Zakuani stabbed a Stephen Dawson cross past his own keeper from eight yards.
However, Peterborough substitute Nathaniel Mendez-Laing equalised just four minutes after coming on. His first involvement was to rise highest to meet a Grant McCann corner from the right and head the ball into the roof of the net from 12 yards.
With 10 minutes remaining, Posh striker George Boyd squandered a wonderful chance to give his side all three points when he stabbed the ball wide from six yards with the goal at his mercy, allowing Jarvis to steal the headlines.
Source: PA
Source: PA