Swapped out the not-really-that-old Denso O2 for a new Denso O2. I've never installed an O2 in the OEM exhaust header, but found it to be cross threaded on the entry. Trying to get it in, it would bite for quarter or half turn and then pop out.
What should have been less than 20 min took me about an hour and a half. Turns out I have the correct 18mm 1.5 pitch tap for the O2 sensor, from back in my honda days. The tap went in pretty easy, so it didn't do much. But what it did do is demonstrate the angle the O2 needed to be to bite the correct threads.
Don't know if it fixed anything yet, because that extra time the job used up was all the time I had to drive it.
I'll drive it tomorrow and see how it looks. I am debating whether or not to reset the ECU, or see if the tables re-learn. Assuming the old O2 was going bad...