In the province's front row, Bryan Young returns at loosehead prop with Justin Fitzpatrick omitted from the team and Declan Fitzpatrick added as cover. New Zealander Grant Webb is named as the back row cover on the bench.
The Ulster backs remain unchanged apart from Ireland's Isaac Boss getting the nod at scrum half ahead of Kieran Campbell.
Commenting on his team selection, Williams said: "We have some guys back starting for us including Bryan Young, Kieron Dawson and Matt McCullough. We are fortunate that we have those options to move things around and keep things fresh.
"Paul Marshall picked up a groin injury against Munster so Isaac will be starting. Our back-line functioned well offensively against Munster and the set play defended well so that unit remains the same with Kieran Campbell in as cover on the bench.
"Grant Webb deserves a shot. I haven't seen him play since I arrived so it is an opportunity for him on the bench, same with Roger Wilson who has had 20 minutes of rugby in last three months."
After losing heavily to Munster last Saturday, Williams knows that the Ospreys, who have qualified for the EDF Energy Cup final, will give his side another stiff test this weekend.
"We know that this will be a tough game, we always said the Munster and Ospreys games would be the two hardest away games but it is a chance to give the young guys some game time and the game will be really important as part of the learning curve as we build up to the home Connacht game on April 11," the Australian added.
Ulster's next home match will see them face Connacht on Friday, April 11. Tickets are on sale now and are available from www.ulsterrugby.com, the Ravenhill ticket office, the ticket line (02890493222 Option 2) from 10am to 12 noon, online and from all usual ticket outlets- a full list can be found at the Ulster Rugby website.
ULSTER (v Ospreys): Bryn Cunningham; Tommy Bowe, Andrew Trimble, Paddy Wallace, Mark Bartholomeusz; Niall O'Connor, Isaac Boss; Bryan Young, Rory Best (capt), Tom Court, Justin Harrison, Ryan Caldwell, Matt McCullough, Kieron Dawson, Stephen Ferris
Replacements: Nigel Brady, Declan Fitzpatrick, Grant Webb, Roger Wilson, Kieran Campbell, Rob Dewey, Mark McCrea.