John Obi Mikel marked his 300th game for Chelsea by scoring the first of two second half goals against Championship promotion contenders Derby County. Oscar rounded off victory for the visitors at Pride Park as they advanced in the FA Cup. Both sides presented early danger with Blues skipper Michael Essien rolling back the years by breaking into the box from deep. Rams forward Jamie Ward meanwhile saw an effort deflected wide after good holdup play by Chris Martin. Samuel Eto'o looked to be in behind County on 20 minutes, but Liverpool loanee Andre Wisdom got back at him superbly to snuff out the danger. Ward continued to cause problems at the other end with Gary Cahill blocking two further efforts. Ramires curled wide for Chelsea from long range on half an hour. Fellow Brazilian Oscar followed suit with a whipped free kick and a second effort from open play. John Eustace then shot straight at Mark Schwarzer in the visiting goal. Derby continued to frustrate the Blues after the break, so Jose Mourinho brought on Eden Hazard and Fernando Torres. The breakthrough came from unlikely source Mikel 25 minutes from time, however, when he headed Willian's near post free kick home. Oscar then doubled Chelsea's lead emphatically as the Rams were undone from a simple throw-in. Hazard, who the free kick for the opener, teed up the Brazilian to blast past Lee Grant. Torres later fluffed his lines after rounding the keeper with the goal gaping. The Blues' Spanish striker then worked Grant with a low effort across goal. Ramries also saw his deflected shot clip the crossbar with ten minutes remaining. Martin sought a consolation for County, but drew a fine stop from Schwarzer late on. Derby sub Mason Bennett also dragged wide. McClaren's men can take great credit for the way they kept Chelsea at arm's lengths for long spell. Mourinho marches on and will face Stoke in the next round.