create a planar surface and use it for coincident
then hide the planar surface
In addition to what Kenneth said, you should create a reference axis to use for your concentric mate.
In my o-ring parts, use a top and bottom reference plane for mating because tangency on an annular surface is very unstable.
If your Interested...
If have a template attached for an O-Ring Used in a compressed state. I use this in Assemblies.
This allows me to adjust 1 sketch for the ID and OD of the groove and Groove Depth.
I then save the item as a Parker # ex. 2-113 for use on that job. (Property Values also need to be updated for the BOM)
You can insert this into position with a Top Plane and Axis Mate.
O-Ring (Master).SLDPRT.zip 95.6 KB