[mactec54]

I looked at the AB Tools 'without hole drop' version. If I understand it correctly, I could do what you suggest and choose a slot milling toolpath first (with a width that matches the major diameter of the O ring end mill), and then I am guessing there needs to be two more passes made at depth that will cut the angle on each side of the slot. Is this correct?

Yes, that is correct just make sure you move the cutter back to center before you move Z axis up, a cheaper option you can also use small dovetail cutter but won't have the radius corners, they work quite well for not so important Oring grooves.

[TMToronto]

Thank you for those tips and options.