The 23-year-old has featured on seven occasions so far this season but only four of those have been in the RaboDirect PRO12, seven less than when he made a real impression in the first-team ranks in 2011/12.
Rees has had to battle with the likes of Lions veteran Adam Jones and Wales cap Aaron Jarvis after the latter's recovery from long-term injury, while the emergence of 20-year-old Dan Suter has also limited his senior opportunities.
"I'm sorry to be leaving my home region, but excited about a new opportunity at Worcester," said Rees, who came on as a replacement in Friday night's LV= Cup win over Sale Sharks.
"I couldn't have learnt my trade in a better place, with the Ospreys renowned for developing props. It's the natural thing in rugby that you are always playing for your future, and with so many young players coming through it was important that I took my opportunity when I had it.
"Unfortunately, that hasn't be the case and with others now coming into the squad my chances have been limited. This move will give me a fresh start."