But he has managed just two goals this term, and has moved to Whaddon Road in a deal initially worth £20,000 with a further £5,000 if the Robins stay in League One.
"Paul played a significant role in our promotion, and we will always be grateful to him for that," said Orient boss Martin Ling.
"But it hasn't quite happened for him this year, and I felt we needed to shuffle the pack in terms of our forwards during the transfer window."