If this can be done, I cant find out how for the life of me. Say I have a permission called mypermission.use and I want to grant it to all groups. Something like "oxide.usergroup grant * mypermission.use" or something like that.

Is there a way to grant all groups a permission at once?

It can be done programmatically but I'm not sure I see the utility.  Granting permissions to everyone is what the "default" group is for.

All players will always be in the default group, so granting to that group would be the same as what you want.

Ahh just set it for default, great point! Ty Wulf. Mark solved please.