What was your net income from the CA rental property?

"Taxable income" shows as $15.5k in the software

Could it be that this amount of income isn't taxable?

(or)

Could it be because Oregon and California have a reciprocal agreement for income tax? i.e. so Oregon taxes me on that amount instead of California?

No one can see your tax return and answer your question precisely. In general, as a California nonresident, you're taxed only on your California source of income. If your income is less than standard deduction and exemption, you do not have to pay taxes and will receive your withholding back. 

For additional information, please, follow the link https://turbotax.intuit.com/tax-tips/irs-tax-return/2017-tax-reform-legislation-what-you-should-know...

What are the California Standard Deductions? https://www.ftb.ca.gov/individuals/faq/net/902.shtml

Could it be because Oregon and California have a reciprocal agreement for income tax? i.e. so Oregon taxes me on that amount instead of California?

They do not have a reciprocal agreement.  Did you fill out your CA non-resident taxes first?  What is your total AGI, including the rental?

Since you have the PC product, go into Forms mode, and check your CA return carefully.  Make sure the rental income is properly allocated to CA on Schedule CA (540NR) in column E.