It took just three minutes for the Derby fans to start choking on the football served up for them as Grimsby exposed the uncertainties in the Rams rearguard.
A clever flick from Iain Ward set Terry Cooke scampering down the right and his driven low cross shot through the hands of keeper Lee Grant to allow Oster to nod the ball home from six yards.
The goal promptly rose the Rams from any post-turkey slumber and midfielder Rob Lee twice threatened an equaliser with thumping long-range drives.
It took an error by veteran defender Steve Chettle to hand Derby a gift of an equaliser.
Chettle's clearance was blocked by Adam Bolder and the rebound fell to Lee Morris who ran 50 yards with only Grimsby defender Georges Santos for company. When the defender backed off into the penalty area Morris curled a left-foot shot into the top corner.
Rams boss Gregory attempted to throw caution to the wind when he introduced attacking substitutes Georgi Kinkladze and Izale McLeod but his team were rocked when Oster swung in a clever free-kick from the edge of the penalty area which beat keeper Grant at his near post.
The Mariners left their hosts all at sea in the final moments when Oster won the ball from Lee in controversial fashion, before feeding Soames who wriggled between two defenders to prod the ball home.
The Derby players surrounded the referee believing Lee had been fouled and to compound their misery the midfielder was stretchered off in apparent agony.