The Rocket won PTC1 earlier this season and took the £10,000 top prize again at the seventh event in the series, which was named the Kay Suzanne Memorial trophy.
O'Sullivan has not won a full ranking event for over two years and has slipped to No 14 in the world, but has thrived in the PTCs and leads the Order of Merit.
Breaks of 95 and 76 saw him take a 2-0 lead in the final before Welshman Stevens pulled one back by winning a scrappy third frame on the colours.
A run of 53 put three-times World Champion O'Sullivan 3-1 ahead then Stevens showed his class with a 127 to win frame five. But any hopes of a fight-back were short lived as a smooth 88 from O'Sullivan in the next gave him the title.
All players wore pink shirts at the event to raise money and awareness for Breast Cancer Care.
The PTC is a series of 12 short events, with the top 24 on the final Order of Merit going through to the Grand Final in March, providing they have played in at least three domestic events and three overseas. The next tournament is PTC8 in Killarney, Ireland, which starts on October 20.
Results (last 16 onwards)
Ronnie O'Sullivan 4-2 Stephen Maguire
Mark King 3-4 Liu Chuang
Andrew Higginson 3-4 Michael White
Joe Perry 0-4 Kyren Wilson
Ali Carter 3-4 Matthew Stevens
Liang Wenbo 2-4 Shaun Murphy
Graeme Dott 0-4 Jamie Jones
Li Yan 3-4 Jimmy Robertson
Ronnie O'Sullivan 4-1 Liu Chuang
Michael White 4-1 Kyren Wilson
Matthew Stevens 4-2 Shaun Murphy
Jamie Jones 2-4 Jimmy Robertson
Ronnie O'Sullivan 4-2 Michael White
Matthew Stevens 4-0 Jimmy Robertson
Ronnie O'Sullivan 4-2 Matthew Stevens
Ball by ball scoring and results from tournaments