I live and work in Oregon - but my employer sent me (back and forth each week) to CA to work with the branch there for 3 months. Do I need to file OR and CA state taxes?

[DoninGA]

Did you have both Oregon and California state taxes withheld from your wages?
Since you live and work in Oregon you definitely have to file an Oregon resident state tax return.

[Glayzner]

No - I was issued a single W2 and the only W/H was for Oregon.

[Glayzner]

I have never filed anything but the standard simplest taxes, this is very new territory for me. Do I just estimate what portion of my salary was earned in CA?

[Hal_Al]

Yes, if you decide to file a CA return, you make your best estimate of the amount from your travel records.

[Hal_Al · Accepted solution]

Technically, you need to file  for both states, but "nobody" does, unless your employer withholds for CA.  See http://money.cnn.com/2013/06/28/pf/taxes/business-traveler-tax-threat/  If you live in a state without an income tax (e.g. WA, FL or TX), it’s more likely you should file in the work states. You can't use the "it all comes out even" rationale for not filing.

If you file, you would file a non-resident CA return and a full year resident OR return. OR will tax all your income for the year. CA will tax only your income earned while physically in CA,  but give you a credit, or partial credit, for the tax you pay OR on the same income*.

*CA & OR are "reverse" or "contrary" states. In most states, your resident state gives you a credit for the tax paid to the non resident state.