O’Hara had covered his first seven holes of the final round in four-under but he could only pick up one more birdie coming home in a 67 as Forsyth held on.
Thomas Higson finished third on eight-under while Graeme Robertson, the newly crowned Loch Lomond Whiskies Scottish PGA champion, shared fourth on seven-under.
O’Hara’s enduring consistency was highlighted again in 2023 as he regained the rankings crown he lost to Craig Lee last year.
In eight Order of Merits events, O’Hara finished fourth or better in five of them as he accumulated 770 points during the season.
Robertson was second on 710.50 points with Forsyth finishing third.