Rees, 32, has also worked with the WRU as a National Skills Coach for the past three years and has previous experience with the coaching of national age grade sides.
Ospreys Elite Performance Director Andrew Hore says there will be more back-room appointments at the region, including a new conditioning coach.
"The coaching staff will be bolstered further," the Kiwi told BBC Sport. "It's going to be different to last year, there will be a lot bigger staff. There will be other in-house appointments, and we are bringing in an in-house coaching development programme."
Hore was brought in from New Zealand by the big-spending Ospreys midway through last season and began a wide-ranging review review of the organisation.
Coach Lyn Jones left the region in May, and Hore has confirmed that assistant coaches Sean Holley and Jonathan Humphreys will lead the Ospreys into next season, with the hunt continuing for a big-name overseas coach to take the top job.