An older man barges into me. While I think he could have prevented that, I don't assume he had any bad intentions. He then apologizes and I want to accept the apology, but how?

Just saying "It's OK" doesn't feel right for me, since if everything is OK, there would be no reason to excuse him. But I also don't want to come across as strange or be impolite/rude.

I just want to inform that person that I accept the apology without phrasing it in a way that won't imply that there was/is no kind of problem.

There actually is nothing that bothers me there, but saying something along the lines "It's OK" or "No problem" would not be an option for me in a situation like this.

So how should I communicate something that preferably gives the feeling to that person that I said "It's OK" under the previously mentioned constraints?

[I have Asperger's](https://en.wikipedia.org/wiki/Asperger_syndrome) which means that, in this very specific context, I feel uncomfortable with expressing something that feels untrue for me.