After a low-scoring first half, tries by Webb and Ben John rendered Chris Fusaro's effort as little more than consolation leaving Ospreys in prime position to clinch a home semi-final when they take on Connacht next Saturday.
That could prove pivotal as no team has ever won an away semi-final in PRO12 history but the job is far from done as Glasgow in second and Munster in third are level on points with the table-toppers, while Ulster are just one point back.
The Irish pair met at the Kingspan Stadium on Saturday and it was Paddy Jackson's heroic conversion with the last kick of the game that earned the hosts a 23-23 draw.
With little to separate the sides, Munster's Keith Earls crossed for seemingly the winning try with four minutes remaining but Paul Marshall dotted down in the corner as the clock seeped into the red and Jackson held his nerve from the sideline.
Elsewhere, Leinster ensured they will be playing top-tier European rugby next term as Jamie Heaslip's early try was the highlight of a scrappy 10-0 victory over Benetton Treviso.
Treviso's Italian counterparts Zebre fared even worse as John Cooney ran in two of six Connacht tries in a 40-10 triumph while Hugh Gustafson touched down for the first time this season as Newport Gwent Dragons beat Edinburgh 19-5.
And Wales hooker Ken Owens was the beneficiary of an inexorable Scarlets driving maul for the only try of their 16-6 victory over Cardiff Blues, leaving them as favourites to seal a sixth-placed finish.
TRY OF THE WEEKEND
While Paul Marshall's last-gasp try eventually snatched a draw for Ulster against Irish rivals Munster, it was Tommy Bowe's scintillating score that first gave them the lead on the stroke of half-time.
Trailing 9-3 as 40 minutes ticked by, flanker Iain Henderson drew in the defence before producing an exquisite offload out of the back of his hand to Bowe and the speedster turned on the afterburners to sprint down the sideline and cut inside the full-back for the score.
BREAK OF THE WEEKEND
Connacht were mostly in cruise control in their 40-10 win over Zebre, but Kieran Marmion's break for their fifth try certainly turned things up a notch.
Deep in his own 22, Marmion demonstrated his blistering pace to burst through Zebre's backline before eventually offloading to Tiernan O'Halloran on halfway, who raced in for the score.
PLAY OF THE WEEKEND
Ospreys put themselves in pole position for that all-important GUINNESS PRO12 home semi-final by seeing off the Warriors and it was Rhys Webb's quick feet that proved the difference to notch their second try.
A clever short line-out saw Scott Baldwin find Nicky Smith who offloaded to a charging Webb and the scrum-half expertly chipped over the oncoming Nikola Matawalu before collecting and dotting down.
PLAYER OF THE WEEKEND
Connacht's John Cooney notched a try double while CJ Stander further enhanced his reputation as one of the Northern Hemisphere's best back-rowers with another commanding display for Munster but it is Josh Matavesi who gets the nod as player of the weekend.
The Ospreys centre showed deft hands to set up Ben John for his side's first try and was at the heart of most attacks as the Welshmen set up a fascinating final round of action.
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