Leaders Exeter Chiefs suffered a third Premiership defeat of the season at Gloucester, while Wasps moved into the top four.
Centre Stuart McCloskey celebrated a century of matches for Ulster by dotting down for the only try against the Ospreys.
Following a head injury against Scotland, Johnny Sexton is fit again and in the running to play for Ireland in Italy.
Wales were dealt a blow on Friday as Leigh Halfpenny's expected comeback for the Scarlets did not come to fruition.
Sio Tomkinson's red card made a Highlanders comeback look unlikely, but Aaron Smith had other ideas and punished the Chiefs.
Eddie Jones sought to dismiss Wales' strong record in Cardiff ahead of England's Six Nations visit, but Neil Jenkins is not worried.