Crescent struggled to subdue Bandon's enthusiasm for the battle until O'Mahony struck with a 50 metre run that yielded the try and put his side firmly in the driving seat.
After that, Crescent never really looked back, although in between early and late second half tries, they fought their corner admirably in a constant rearguard action.
But there was no doubt about the merit of this comprehensive Crescent victory or the quality of the five tries they scored on a heavy pitch that wasn't suited to young enthusiastic players.
Crescent coach Dudley Herbert launched somewhat of a broadside when he remarked: "I think cup games should be played on pitches of better standing."
Still, he was happy with the outcome considering this was Crescent's first outing of the season in the competition.
Alan Jennings kicked Bandon into the lead with a penalty after 15 minutes, Crescent opened their account when speedy full back Mike South flashed through for a 24th minute try and Paddy O'Carroll expertly converted from out wide.
O'Mahony's try came next and Ronan O'Sullivan scored another excellent try in first half injury time both converted by Carroll.
Clem McInerney's midfield break paved the way for a further try from South early in the second half. Carroll converted and then enjoyed his own special moment when he crossed for the fifth try late in the game and completed the scoring by adding the conversion.
Crescent now meet St. Munchin's in the semi-finals.
M. South, P. Carroll, C. Aherne, C. mcInerney, M. Hayes, R. O'Sullivan, D. O'Carroll, R. Leonard, L. McInerney, P. Lally, B. Lynch, O. Keely, S. Keyes, D. McGuire (captain), B. O'Mahony.
Replacements: M. Nicholas for O'Mahony (37, injured), C. Cooney for C. McInerney (48), B. Gavin for Aherne (56), D. Murphy for Leonard, A. Lynam for Keyes (both 60), D. Waters for O'Sullivan, H. Mulcahy for O'Carroll (both 65).
A. Willis, S. Huleatt-James, L. Barry, T. Ryan, C. Culton, A. Jennings, C. Crowley, R. Falvey, S. Deane, P. Stoutt (captain), T. Laubach, A. Lynch, M. Lester, N. Peters, D. Crowley.
Replacements: D. Warner for Ryan (23, injured), J. Coleman for Falvey (55).
K. Inbusch (Munster).