Blackpool, unbeaten away from home since New Year's Day, produced a lacklustre performance with the Millers having the better of proceedings.
Chances were at a premium in the first half with the Seasiders - seeking their second Yorkshire scalp in three days after beating Bradford - spurning a glaring opportunity on 16 minutes when joint top-scorer Scott Vernon lashed his shot wide with his goal at his mercy.
Loan ace Rossi Jarvis curled a free-kick wide for United before Vernon saw his drilled effort comfortably saved.
United, with just one win in their previous eight home matches, went agonisingly close to taking the lead on 24 minutes when Delroy Facey's fierce angled shot was parried by Paul Rachubka.
The Seasiders were in second gear for most of the first half but almost took the lead moments before the interval with Ian Sharps' vital interception denying Andy Morrell after Vernon's assist.
On the restart, Neil Cutler spurned a low shot from Vernon with his feet before the Millers grabbed a deserved opener on the hour.
O'Grady tucked the ball home from close range after polished play from Facey and Ian Henderson dissected the Blackpool rearguard.
Seasiders boss Simon Grayson sent on highly-rated Keigan Parker in a bid to pep up his below-par troops, but United went close to having a second with O'Grady heading wide while Henderson shot straight at Rachubka after being sent clear.
Morrell shot agonisingly wide in a late rally by the visitors, while in stoppage time Ian Evatt saw his header crash against the post as the Millers held on for a deserved win.