Two tries from centre Darren Cave ensured Ulster triumphed in a war of attrition with Ospreys, who lost for the first time in the GUINNESS PRO12 this season.
The home side had established an 18-3 lead at the break, but Ospreys got back to within two points as they ramped up the pressure and conditions deteriorated.
But Cave made the points certain with a last-minute score, as Ulster won their fourth game in a row.
Ospreys had equalled their best ever start to a PRO12 season with seven wins from seven, but the loss of ten players to international duty put paid to Steve Tandy's team's run at the Kingspan Stadium.
Ulster are imperious at home, with just one defeat in front of their own fans since September 2013, and took the lead on ten minutes when Cave, released earlier this week by Ireland, gathered from a ruck on the line and dived over. Paddy Jackson kicked the extras.
The 22-year-old fly-half added two more penalties, but Sam Davies finally gave Ospreys a foothold in the game when he nailed his 30m kick after half an hour.
The league leaders' night went from bad to worse before the interval, with Duncan Jones forced off with an injury to his left knee suffered innocuously at a line-out.
And, on the stroke of half-time, the home side scored again. This time Franco van der Merwe crossed after Clive Ross helped him over the line, though Jackson missed the posts for the first time to leave Ulster with an 18-3 advantage at the interval.
The home side were a shadow of themselves in the second half. And, as indiscipline crept into their game, Ospreys profited.
Martin Roberts started the visitors comeback four minutes after the restart, crossing at close range following a maul. Davies added the extras.
On the hour Davies saw a penalty sail over. And the league leaders moved to within two points two minutes later when Davies' faultless left foot made it four from four.
Ulster fly-half Jackson walked down the tunnel clutching his arm with 15 minutes remaining as Ulster, so dominant in the first half, looked like the pressure had got to them.
But on 79 minutes Cave side-stepped a challenge and went over for his second try after Ulster enjoyed a rare period of pressure, much to the relief of the vociferous home crowd. Replacement fly-half Ian Humphreys' kick completing the win.
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