The dealer will probably tell you when your car was serviced it was test driven after and everything was is normal working order when it left the shop and they can't be responsible for what happens to your car unless it was caused by something they did.

When you picked up the car did you make them aware of what you noticed? You would have to prove your problem was caused by the oil change and that would be next to impossible.

Good luck hope its not serious

PS If I'm not mistaken the O rings which you speak about help supply oil to the lifters and if defective you lifters would clatter.
Did you notice noisy lifters?