Wednesday almost had the lead inside the first three minutes, but Andy Lonergan produced a save to deny Adthe Nuhiu after a looping cross had found the striker at the back post.
The ex-Watford forward almost had a second shortly after, but this time he could only head wide from another set-piece delivery.
Forestieri was involved yet again before the half-hour mark, and Wednesday had a penalty appeal turned down after the forward went down inside the area under a challenge from Richard Stearman.
Ryan Tunnicliffe came close to restoring parity shortly before the half-hour mark when he flashed an effort just wide from outside the area.
Wednesday had the advantage once again when Tom Lees struck towards the end of the half.
However, the visitors reduced the deficit on 67 minutes when Tom Cairney converted from close range after a teasing delivery from O'Hara on his unfavoured right foot.
The Cottagers were unable to find an equaliser though, and the home side move up to 14th in the Championship table.