The 21-year-old impressed with the Os during a loan spell towards the end of the 2006-07 season and Brisbane Road boss Martin Ling is delighted with his latest signing.
He told the club's official website: "I'm very pleased to have Ryan back at the football club. He had a massive impact when he was here two seasons ago and his goals went a long way to keeping us up.
"We raised the possibility of taking him at that time, but were quoted a £250,000 fee by Norwich.
"He was always high on my list of priorities this summer and it's great to have him back."