Williams saw off Martijn Kleermaker of Netherlands, Martin Heneghan who represents Republic 0f Ireland and teammate David Arwyn Morris before thrashing Neil Duff 5-1 in the final.
O'Sullivan on the other hand saw off Linda Harte of Republic of Ireland, fellow teammate Ann Marie Potts before seeing off Chris Savvery in an all Welsh final.
Click here to view the full schedule for the Six Nations Cup.
Last 16
Wessel Nijman (NED) 4-2 Trevor Vallely (IRE)
Neil Duff (NIR) 4-3 Nick Kenny (Wal)
Paul Milford (ENG) 4-3 Scott Mitchell (ENG)
Jeffrey Sparidaans (NED) 4-1 Nigel Heydon (ENG)
Jim Williams (WAL) 4-1 Martijn Kleermaker (NED)
Martin Heneghan (IRE) 4-3 Darren Bingham (WAL)
David Arwyn Morris (WAL) 4-3 Andrew Ryan (IRE)
Martin Atkins (ENG) 4-1 Alan Soutar (SCO)
Quarter-Finals
Neil Duff (NIR) 4-1 Wessel Nijman (NED)
Paul Milford (ENG) 4-2 Jeffrey Sparidaans (NED)
Jim Williams (WAL) 4-1 Martin Heneghan (IRE)
David Arwyn Morris (WAL) 4-2 Martin Atkins (ENG)
Semi-Finals
Neil Duff (NIR) 4-0 Paul Milford (ENG)
Jim Williams (WAL) 4-1 David Arwyn Morris (WAL)
Final (Saturday)
Neil Duff (NIR) 1-5 Jim Williams (WAL)
Quarter-Finals
Ann Marie Potts (WAL) 4-3 Lorraine Winstanley (ENG)
Rhian O’Sullivan (WAL) 4-1 Linda Harte (IRE)
Aileen de Graaf (NED) 4-0 Sharon Prins (NED)
Chris Savvery (WAL) 4-3 Susanna McGimpsey (SCO)
Semi-Finals
Rhian O’Sullivan (WAL) 4-3 Ann Marie Potts (WAL)
Chris Savvery 4-1 (WAL) 4-1 Aileen de Graaf (NED)
Final (Saturday)
Rhian O’Sullivan (WAL) 5-0 Chris Savvery (WAL)
https://twitter.com/NDBdarts/status/1142493692000948224
https://twitter.com/NDBdarts/status/1142500160825700352