Disciplinary council representative Anais D'Arville argued O'Donovan's case was more serious than the now-Melbourne Victory coach's in the sense that it was deliberate and did not occur in the run of play.
It was less serious, he argued, in that Manny Muscat was not hurt, whereas Zahra sustained a serious knee injury.
Mariners chief executive Shaun Mielekamp said the club would look to appeal the hefty suspension, which he said was over the top.
"We are extremely disappointed at the decision," Mielekamp said.
"The disparity in sanctions issued for this match highlights the flaws in the system.
"The media circus around the incident has had a high impact on the result and we will look at our various avenues for appeal."
While Muscat received a two-match ban for his part in the incident, O'Donovan will not be eligible to play until round 22 in a massive blow to the already last-placed Mariners.
Only Danny Vukovic has received a bigger suspension in the A-League, the former Mariners custodian copping a nine-month ban for striking a referee in 2008.
Central Coast did not bring a lawyer to the two-hour hearing and was represented only by O'Donovan, Mielekamp and coach Tony Walmsley, who spoke in defence of his striker.
O'Donovan apologised, citing a "momentary lapse of mental faculties" and said he should have been sent off.
He and the Mariners argued the violent headbutt was an instinctive, almost involuntary reaction while in a dazed state from the strike seconds earlier, which had caused his eye swell.
O'Donovan said he could sense and hear Muscat approaching him from behind swearing, and had "leaned over" in self-defence.
The committee disagreed, of the opinion O'Donovan's pronounced step towards Muscat just before indicated it was a voluntary act.
"It's going to be a stigma for the rest of my career, I have no doubt about it," O'Donovan said.
"If I could take it back, I would ... it's been the most stressful week in my football career."
O'Donovan also claimed the subsequent media coverage of the incident had "muddied the waters" and made him out to be "some sort of animal".
A-LEAGUE'S BIGGEST BANS:
Danny Vukovic: Nine months and three months suspended, striking a referee.
Tiago Calvano: Eight games and four suspended, holding a referee.
Kevin Muscat: Eight games, serious foul play.
Roy O'Donovan: Eight games, headbutting.
Ney Fabiano: Six games, spitting.
- AAP