It is also possible that you simply did not have enough withheld when you combined the W-2 you forgot with the rest of your income. Oregon has a graduated tax system. This means the more you make, the higher percentage you pay. So when you added more income it is possible that it increased your tax rate causing you to owe.
Also, if you amended your return, and now owe, it doesn't necessarily mean that you OWED taxes to start with. If you added income to your Oregon return, and you had received a refund, it is possible that increased income upped your tax liability and that you had too big of a refund so you are just returning some of the refund you were not supposed to have and that you wouldn't have had if you would have included the W-2 on the return when originally filed.