RE: Then 7 payments are taxable to CA and 5 payments are taxable to OR.

Except that, IF  the return was done by @IRonMaN , he would ask: "what about the 13th month?"

Anyway I just wanted to add this: in this scenario, the client would be filing two "Part-Year returns". 

For CA, it would be on F540NR.  Fill out Schedule CA (540NR).  You're on your own for OR.