The 30-year-old, who plays for Central Coast Mariners, was cited for his attack on Wellington Phoenix player Manny Muscat over the New Year period.
Only former Mariners goalkeeper Danny Vukovic has received a bigger suspension in the A-League with a nine-month ban for striking a referee in 2008.
O’Donovan pleaded guilty to violent conduct. Muscat was earlier banned for two matches for his part in the confrontation.
The Mariners said they were “extremely disappointed” and may appeal against the decision.
“The disparity in sanctions issued for this match highlights the flaws in the system,” club chief executive Shaun Mielekamp said.
“The media circus around the incident has a high impact on the result and we will look at our avenues for appeal.”
O’Donovan is in his first season with the Mariners after stints with Sunderland, Coventry City, Blackpool and Northampton Town in England.