"The current set up is going from strength to strength but I need to make a move at this stage of my career as I want to push as hard as possible for a place in the Scotland squad for next years World Cup and I think playing in a new and challenging environment will give me that opportunity."
Reivers head coach Steve Bates said: "Nikki has been a great servant to the club and I wish him well for his future with The Ospreys."
Nikki has been with the Reivers since the team was set-up in 2002 after winning the BT Cellnet Cup with Hawick the same year. He has played in 16 of the Reivers' 19 games this season so far scoring eight tries in both domestic and European competition. He has represented Scotland at all levels and has 3 Scotland caps. Nikki scored his only try for Scotland in the victory over South Africa in 2002.
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