I would like to make a groove to receive an O-ring on this part. To do this, I think I have to create a plane on the axis of the hole, to make the sketch, but I can't do it.
When I want to create this plane, I select the axis and then reference geometry and plane, so I have the axis as a first reference but then I pedal...
create a sketch on the top of your piece and then in it you put a dot on the int or ext edge (then if you move it, you can constrain it on one of the quarters of the diameter) of the diameter; or a line that starts from the middle (where you already have your axis towards the outside of your piece and you can also constrain in the same way as the point). Once done, you can create your plan by first selecting your axis and then the sketch you just created and then it will work. Unless you already have shots in your room, then you will have to shift it to the axis you have in the middle of the room...
You have an example in the attachment, you just have to select your temporary axis of the part and validate, you will have your plan at the level of your axis.