Petrasso, 19, will join Orient until October 7 and will be available to play in cup competitions.
Petrasso told the official club website: "Orient did very well last season and caught my eye and I just want to help here in anyway I can.
"I've had two loans in League One before which were great for me. This season I want to keep getting games under my belt and keep improving."
Petrasso has made just one appearance for the Premier League outfit but has had spells on loan at Coventry City and Oldham Athletic.