Fly-half Simon Humberstone scored 11 points on his first start for the Cardiff Blues in this win against the Ospreys in the LV=Cup.
Humberstone converted the Blues two tries scored by lock Macauley Cook and hooker Kristian Lacey and slotted over a penalty and a drop goal for his side.
Captain Jonathan Spratt scored the Ospreys only try of the game.
The Ospreys were 13-11 ahead at half-time but the Blues fought back to clinch victory.
Ospreys: Jamie Murphy, Aisea Natoga, Tom Isaacs, Jonathan Spratt (c), Dafydd Howells, Matthew Morgan; Tito Tebaldi, Marc Thomas, Matthew Dwyer, Dan Suter, Lloyd Peers, Rhodri Hughes, Graham Knoop, Arthur Ellis, Joe Bearman
Replacements: Evan Yardley, Nicky Smith, Nicky Thomas, Sam Williams, Sam Lewis, Rhys Webb, Sam Davies, Ben John
Cardiff Blues: Dan Fish, Richard Smith, Gavin Evans, Dafydd Hewitt, Chris Czekaj, Simon Humberstone, Alex Walker; Sam Hobbs (c), Kristian Dacey, Benoit Bourrust, Macauley Cook, James Down, Ellis Jenkins, Thomas Young, Robin Copeland.
Replacements: 16 Rhys Williams, 17 Taufa'ao Filise, 18 Tom Davies, 19 Miles Normandale, 20 Rory Watts-Jones, 21 Luke Hamilton, 22 Lewis Jones, 23 Tom Williams