I know that in Oregon we can assess a late charge on a
commercial loan 10 days after the due date. In Washington, it's 10 days for
consumer and commercial loans. What is the ruling if the borrower is an
Oregon corporation, their headquarters are in Washington and the
collateral is real estate in Washington?

I would think the question will be decided based upon your loan documents. Is there a clause in your loan documents that says Oregon or Washington laws govern? Whatever the loan documents say should be your answer, regardless of where your borrower is organized/registered.

The 1 potential "wild card" is that, w/property, sometimes the law of the state in which the property is located will "trump" what your loan documents say on some issues.

Im not sure this is a real concern in your situation, because (if I understood correctly) your question deals more w/the right to assess and collect a late charge, not the ability to either (i) collateralize the late charge w/the property or (ii) collect the late charge from the forced sale of the property.

I recently researched this issue myself. When doing business in multiple states, you "import" your own late charges under the definition of interest. BTW, we are an OCC bank. See 12 CFR 7.4 in regard to preemption and definition of interest.