I'm not sure. I found out about DH's affair 18 months ago and since then we have been rebuilding things after a brief time apart.
I suppose that at the end of the day, an apology from the OW (a work colleague) is not really relevant to me because the whole point is to rebuild the marriage without the OW. The actions that need to be analysed, explained and prevented in the future are the actions of the cheating DH. Although in my case I think that the OW should take some of the blame becuase it is against "humanity" to enter into a relationship with a man who you know has a 1yo and a 3yo and a wife.
I wouldn't really mind if the OW apologised (although it would be awkward in person). However, I also feel that if the OW is sorry to me, she should be even more sorry to my totally innocent children so maybe an apology to me is actually worthless?
The person an OW needs to be most sorry to is herself. If you have been the OW, perhaps concentrate on that. If you knew that the man was married, then you have let yourself down by having a relationship with him.
With every passing day, the OW becomes less and less relevant to me. Also you have to consdier whether the OW is apologising to benefit herself. And also sorry is just a word. One day, the shit is going to hit the fan. My DC are going to find out what their dad did to us when we were very vulnerable because 25+ family members know and it won't be able to be kept quiet. The OW can apologise to me now, but me and DH will have to face that in the future.
This is all a bit garbled, but I hope some of it might be helpful.