MySQL - Averaging values from two possible columns

Question

I have a table that looks like this:

ID      SCORE     OVERRIDE_SCORE
1       15        NULL
2       16        17
3       16        NULL

I am averaging the scores using:

SELECT AVG(SCORE) FROM QA
WHERE SCORE is not NULL

I would like to use the OVERRIDE_SCORE as the SCORE so that when the override score was present, the average would use that value instead of the value in SCORE.

Is that possible in a query?