Clarification. Upon completing my extension, the IO reminded me I would still need to do my 90-day reporting, which had an appointment/due date of about 3 weeks after my extension - which was my first extension based on an OA Visa based on being over 50. I now recall that I did that 90-day reporting in person, only because I found it easy to do since I live close to immigration. After that, I completed the next one online without any problems. The timeline or due dates of my 90-day reporting never changed due to getting my extension of stay renewed.
I have successfully completed an online report after an extension of stay, and the timeline I used was based on my prior appointment or due date for 90 day reporting, with no change due to the date of my extension to stay. I have subsequently completed one 90 day report via EMS post, which I mailed 10 days before that due date, which worked fine.