Ospreys host Harlequins at Brewery Field this Sunday with both teams still in with a chance of qualifying.
The home side must win and hope other results go their way; Harlequins need just two points to secure qualification.
Rhys Webb will skipper an Ospreys team that has been hit by injuries and international call-ups.
They do include Wales internationals Jonathan Thomas and five-time capped Kiwi Campbell Johnstone.
Rory Clegg returns at fly-half and Ross Chilsholm starts at fullback after coming back from injury last week.
Luke Wallace will lead the side as they aim for a home semi-final.
Ospreys: 15 Jamie Murphy, 14 Tom Grabham, 13 Jonathan Spratt, 12 Ben John, 11 Dafydd Howells, 10 Matthew Morgan, 9 Rhys Webb (c), 8 Morgan Allen, 7 Arthur Ellis, 6 Jonathan Thomas, 5 Sam Williams, 4 Lloyd Peers, 3 Campbell Johnstone, 2 Scott Baldwin, 1 Marc Thomas.
Replacements: 16 Matthew Dwyer, 17 Duncan Jones, 18 Dmitri Arhip, 19 Ian Gough, 20 George Stowers, 21 Dan Baker, 22 Tom Habberfield, 23 Kah Fotuali'i.
Harlequins: 15 Ross Chisholm, 14 Seb Stegmann, 13 Matt Hopper, 12 Ben Botica, 11 Sam Smith, 10 Rory Clegg, 9 Karl Dickson, 8 Tom Guest, 7 Luke Wallace (c), 6 Joe Trayfoot, 5 Charlie Matthews, 4 Sam Tworney, 3 Will Collier, 2 Dave Ward, 1 Mark Lambert.
Replacements: 16 Henry Allen, 17 Darryl Marfo, 18 James Johnston, 19 Charlie Walker, 20 Maurie Fa'asavalu, 21 Jordan Burns, 22 Louis Grimoldby, 23 Ollie Lindsay-Hague.
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The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.