O'Byrne, 28, has one goal and four points along with 54 penalty minutes in 34 games this season. Last season, the 6-foot-5, 234-pound right-handed shot led Colorado in blocked shots and was second in hits while playing a career-high 74 games.
A third-round selection of the Montreal Canadiens in the 2003 NHL Draft, O'Byrne has played in 300 NHL games for the Canadiens and Avalanche, compiling four goals, 37 points and 363 PIMs. He has played in 19 Stanley Cup Playoff games.
"Toronto is a team that’s right in the [playoff] mix and I’m excited to be joining them,” O’Byrne told TSN. “From my days in Montreal, I always enjoyed playing in Toronto.
“I was pretty surprised, but my contract is up next year, so I had a feeling something might happen.”
O'Byrne has a salary-cap hit of $1.8 million for this season and can become an unrestricted free agent this summer.
Asked if he knew why the deal was made, O'Byrne replied, "It's tough to say. Obviously in a shortened season, we lost a few in a row and things kind of snowballed there. But there are good young players coming up there and I think they'll be a good team for years to come."
O'Byrne may be paired with John-Michael Liles on Toronto's blue line -- they played together in Colorado in 2010-11 before Liles signed with the Maple Leafs in the summer of 2011.
“He’s going to be one of our eight defensemen,” general manager Dave Nonis said. “He’s not going to come in here and take over the defensive core. He’s a defensive defensemen.
“We feel that he can play with good players, with John-Michael Liles, a [Jake] Gardiner, a [Mike] Kostka. Those are the likely pairings but that will be a [coach] Randy Carlyle decision.”