Although table toppers Ulster still enjoy a comfortable 10-point cushion over the Welsh region, Mark Anscombe's men will be disappointed to have lost their 100% home record at this stage of the league campaign.
Although deprived of half a team's worth of players called up for Six Nations duty with Ireland, Anscombe was nevertheless able to field a strong side which was bolstered by returning captain Johann Muller and centre Luke Marshall.
The 19-year-old Olding made his first senior start at out-half and young prop Andrew Warwick earned himself a berth on the replacements bench alongside, among others, a fit-again Dan Tuohy.
After a lively opening salvo from both teams, Pienaar kicked Ulster into a fifth minute lead at Ravenhill with the Ospreys penalised for not releasing in the tackle.
This score was cancelled out by Matthew Morgan's swerving kick five minutes later after an Ulster offside, but the same player was wildly off target with a close range drop goal attempt in the Ospreys' next attack.
Pienaar added a second penalty from just shy of the halfway line on 15 minutes, but with the Ospreys playing the more adventurous rugby as the first quarter drew to a close, Ulster were lucky to escape without any further concessions.
A promising Welsh attack broke down with flanker Sam Lewis pulled up for holding on too long in the tackle. The Ospreys came straight back, however, and Morgan was on hand to equalise once again with another penalty.
Their next attack threatened to be even more lucrative, with several phases taking the visiting forwards to within a metre of the try-line. But the Ulster defence resisted and Andrew Trimble was eventually able to clear after a hopeful prod forward from Morgan's boot.
Ulster mounted their first serious attack on the half hour mark, with good ground made by Marshall and Paddy Wallace in particular, and swift recycling from scrum half Pienaar.
Suddenly under intense pressure, the Ospreys infringed directly in front of the posts, with Joe Bearman failing to release ball carrier Michael Allen quickly enough for the referee's liking, and Pienaar chipped over to make it 9-6 to the hosts.
With the balance of power now having swung in their favour, Ulster closed out the first half in Ospreys territory. Yet, taking into account the possession and territory over the period as a whole, they could count themselves fortunate to run off at the interval ahead on the scoreboard.
With Darren Cave replacing a hobbling Wallace just minutes into the second period, the game lulled along at a somewhat disjointed rhythm.
The Ospreys again resorted to a drop goal attempt on 50 minutes, this time from Fotuali'i, which went the same way as Morgan's had in the first half.
The out-half successfully kicked a penalty four minutes later, however, to tie things up. An off-colour Ulster lineout finally got going near the hour mark, a strong drive from the forwards promising much before it was frustratingly halted by the referee's whistle.
With Tuohy, Paul Marshall and Niall Annett all entering the fray for Muller, Pienaar and Rob Herring, Ulster seemed reinvigorated by the fresh legs. Pienaar picked up a shoulder injury which will cause some concern.
The home side were rewarded for five minutes of sustained pressure with a penalty in front of the posts, which also saw Ospreys number 8 Morgan Allen sent to the sin-bin for going off his feet.
Olding took over the place-kicking duties in the absence of Pienaar, and calmly slotted over to edge the league leaders back in front at 12-9.
However, the Ospreys threat which had been bubbling under for the entire match then came to a head, prop Ryan Bevington showing great pace and stamina to collect Fotuali'i's angled pass and outstrip the chasing Ulstermen for the try. Morgan added the extras for a sudden four-point advantage.
Ulster responded manfully, heaping pressure on the Ospreys defence with multiple phases and overlaps which the visitors resisted with some aplomb.
With less than five minutes remaining on the clock, an untimely spill from Mike McComish handed possession back to Steve Tandy's charges who then set about containing Ulster in their own half.
A last-minute Ulster lineout on halfway once again went awry and although the hosts regained possession through sheer persistence, ball carrier Lewis Stevenson was soon bundled into touch by a swarming Ospreys defence to bring the match to an end.
The Ulstermen will have an opportunity to get back to winning ways next Friday when bottom side Zebre make their first ever trip to Belfast.
|Ulster Rugby Score Card|
|Ospreys Score Card|