So now I have two visas; a 3-entry tourist visa (bought in a panic post-refusal of non O) plus an O-A newly acquired.

Silly boy, yes I know and obviously money to burn etc etc.

Apart from making it very clear which visa I am entering the country on, any other observations? (The Embassy didn't cancel the tourist visa when they issued the O-A earlier this week)

Just put the OA visa number on your arrival card and be sure the officer uses your OA visa when you get stamped in.

In relation to two visas on entry,

I gave the OP a headache coming in last week. I had a retirement extension valid until 31 December 2018,

But i got a new tourist visa outside 60 day (extendable to 90 day).

Reason, I don't need to renew the retirement extension, I expect to be transferring to a Non-B for business / work permit within a month or two.

I put my new tourist visa number on the entry TM6. The OP pointed out that i had two visa and did not know what to do.

Superiors had to be called and it took a while to get this figured out.

I explained to them I need to stay 2 months.

Why would I want to have to leave again in 18 days or need to renew the retirement extension.

I just want to use the new tourist visa.

For your info, we got it resolved when the chief agreed that i could write over the existing retirement extension in my passport that i did not want to use it and sign. Then the OP stamped me i and I checked that they had identified the correct visa number and exit date.