A large number of Vale players had complained about their treatment under Gannon in a meeting with the club’s directors on Monday afternoon, but there were no signs of disharmony in a keenly contested first-half which the home side edged.
Both sides squandered early chances as Vale goalkeeper Chris Martin made a terrific save to deny Adam Le Fondre before home striker Justin Richards scuffed his shot when clean through.
The Valiants were denied by the woodwork soon after when a 20-yard free-kick by O’Shea came back off the base of Andy Warrington’s right-hand post.
But O’Shea, on loan from Birmingham, made no mistake when he scored his first goal for the club with a stunning strike eight minutes after the break. Doug Loft’s forward pass spun into the air off a defender and O’Shea cracked an unstoppable 20-yard volley.
Justin Richards should have added a second just before the hour when he fired a close range shot over the top after some calamitous Rotherham defending.