O- ring in engine needs to be fixed?

I have a Ford Focus and I was told the o-ring in the engine needs to be fixed. He said he needs to check again to make sure that's the problem. I have no idea what any of that means but could anyone guess an estimate of the cost for repairs? This is all the information I know that he told me so sorry if I didn't explain well
Added (1). He said that there is no pressure from the engine and something is wrong with the o-ring, I would just like to know what the cost would be like

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