Have you still got the old sparkplugs that you've replaced? They may tell a story if you have a look.
What can happen when oil gets in the plugwells is that 'tracking' can occur, which is basically leakage of the spark from the centre contact (top end, not inside engine bit), down over the outside of the ceramic insulator to ground. This typically leaves a black line of carbon vertically down the outside of the insulator, which is the 'witness' mark made up of cooked engine oil and other crud.
Even with new plugs, if there's a matching line of this crud on the inside surface of the rubber boot at the end of the HT lead, you may still suffer trouble under 'difficult' combustion conditions like cold-start or sudden increase in load ( e.g. pulling away from standstill). Can probably be cleaned off carefully if this has occurred.
As Tunnie suggests though, another reason for rough running for a short while from cold-start is that there is coolant in one of the cylinders. This rapidly evaporates or just gets pumped out of the exhaust ports, so resolves quickly. It will get worse though, if this is what is happening, and new headgasket is the only solution.
Not unheard of in high-mileage, less-than-wonderfully-serviced Omega engines.
