O'Hara lashed in a left-foot shot from 15 yards into the top corner of the net after another loan signing, Kevan Hurst, had picked him out after cutting in from the left.
Chesterfield got off to a sound start in their quest to extended their unbeaten record and in the 14th minute Hurst hit the underside of the bar before Wayne Allison opened the scoring.
Paul Hall crossed from the right and Allison was unmarked when he headed home his 11th goal of the season just inside the post.
Bournemouth had chances to equalise with substitute James Coutts volleying at Barry Roche from 10 yards.
The worst miss came on the half-hour when Dani Rodrigues shot over the bar with only the goalkeeper to beat after Shaun Cooper had set him up.
Bournemouth lost influential midfield player Steven Foley-Sheridan with a foot injury and his replacement Coutts was later substituted for Brett Pitman.
Bournemouth were level in the 66th minute when Gareth Davies appeared to haul back James Hayter to concede a penalty.
Hayter stepped forward to score with the spot-kick - his 16 goal of the season and his sixth in the last eight matches.
Then came O'Hara's winner and there was a great chance for substitute Colin Larkin to score Chesterfield's third in the last minute.
Reuben Hazell hit a long ball down field and Larkin was clear with only Gareth Stewart to beat before dragging his shot wide.
Bournemouth pressed hard for a second equaliser with Pitman and Cooper going close but there was no way back.