For my first time ever on my one-year O's I should have reported 90 day about 24 days ago. I completely forgot. I just realized this getting ready to leave from Suvarnabhumi this Monday night.

Do I rush to Immigration Monday morning (I've got LOTS to do that day) and what will happen?

2) Is there somewhere I can do this online?

3) What will the fine be at immigration Monday night? Can it just be taken care of there on the way out?

Since you are late, you have to go in person and pay a fine. Normally they charge 2,000 baht when you report late.

Not likely they will notice at the airport, so you have the option of going Monday and sort it out or going after your return. You cannot take care of it at the airport and raport and pay the fine there.

If you are going out of Thailand before you next have to go to immigration to renew your visa then don't bother and you will save the Bt2000 fine. It seems they never check at the airport. I did this recently, my 90 report was 8 months over due. No problem. But be sure to remove the report slip.

They may still catch it at his next report if they check passport and there is still the matter of unlimited 200 baht per day over and above the max normal fine of 5,000 baht which has not yet been explained on what condition it would be assessed. I suspect only 2k would be assessed but it remains an open question.