Personally my monsters phase right through characters without OA and the players haven't even thought of trying to do an OA.
If you haven't told them they most certainly will believe they are unable to.
Yeah, I know, but they are familiar with the general rule and it is less than obvious that phasing should provoke OA from a logical point of view. Since they don't claim the OA, they are basically agreing with the houserule that phasing moves don't provoke.
By the same logic, however, the PC's shouldn't even be able to hit the phasing creatures with their weapons. I don't understand the disconnect with OA's.Yeah, I know, but they are familiar with the general rule and it is less than obvious that phasing should provoke OA from a logical point of view.