Ospreys: (12) 26
Ospreys lead their pool on try count after beating Calvisano
Tries: R Jones, Byrne, A Jones, Lloyd
Cons: Hook (3)
Calvisano: (3) 9
Pens: De Marigny (3)
Ospreys overtook Stade Francais at the top of their Heineken Cup group with a bonus-point victory over Italy's Calvisano at the Liberty Stadium.
In torrential rain, the Welsh side scored tries from Ryan Jones and Lee Byrne and added touchdowns from Adam Jones and Andy Lloyd after the break.
Calvisano's Roland De Marigny hit three penalties for the visitors.
The win puts Ospreys ahead of Stade on try count with games against the French (home) and Sale (away) still to come.
Ospreys trounced the Italians 50-27 in their first meeting but were made to work harder this time around.
Skipper Duncan Jones went over after just two minutes but referee Wayne Barnes had already blown for a penalty against Calvisano.
But early home pressure produced a try for number eight Ryan Jones in the fourth minute, though James Hook was wide with the touchline conversion.
It wasn't our greatest performance - we had to raise the fellows and it was nothing more than a sound performance
Ospreys enjoyed all the early advantage but were unable to add to their tally and Calvisano narrowed the gap with a penalty from De Marigny, who earlier missed from 50m.
Three minutes before half-time the Ospreys grabbed a second try when Byrne burst over in the right corner from a scrum on the left. Hook's conversion gave the Welsh region a 12-3 interval lead.
The home side increased their advantage with a try from prop Adam Jones just two minutes into the second half.
But the Ospreys lost their verve and De Marigny hit two penalties to give his side hope - the first after Jonathan Thomas was sin-binned for a professional foul.
Thomas was followed in to the bin seven minutes later by Calvisano replacement Ben Gissing, and it took the Ospreys until the 65th minute to secure the all-important fourth try and the bonus point when Lloyd squeezed over.
Ospreys coach Lyn Jones:
"It wasn't our greatest performance and these games can be quite hard. We had to raise the fellows and it was nothing more than a sound performance.
"In the first half we were a little lateral but we got our focus a bit more in the second half.
"It is one of those performances we need to put a line under and forget about. We got what we wanted out of the game and we have to move on.
"I wasn't getting twitchy about the bonus point and we are happy with the four tries, but I guess everybody was expecting 50-plus points.
"At this stage we usually talk about other teams losing to help our cause, but this season we are talking about beating Stade Francais at home and setting up the final weekend (at Sale).
"All we can do is put in a good performance against Stade as we did last year."
Ospreys: L Byrne; N Walker, A Bishop, J Hook, S Williams; G Henson, J Marshall; D Jones (capt), H Bennett, A Jones, A Wyn Jones, I Evans, J Thomas, S Tandy, R Jones. Replacements: B Williams, C Griffiths, B Cockbain, A Lloyd, M Roberts, S Connor, J Vaughton.
Calvisano: M Cagnoni; M Pratichetti, C Zanoletti, J Downey, W Spragg; R De Marigny, P Patelli; A Vigne Donati, L Ghiraldini, R Moore, M Ngauamo, V Bernabo, M Zaffiri, A Persico, D Dal Maso.
Replacements: S Silva, G Intoppa, B Gissing, N Cattina, P Griffen, D Pastormerlo, M Gallinetti.
Referee: Wayne Barnes (England).