I may want to be out of Thailand long enough to miss any conceivable window for extending my 12 month permission to stay on my O visa for retirement. Will applying for another O visa be easier, harder or no different now that my passport has an expired O visa and 5 years of extensions? I'll also have a Thai bank accound and a leased Thai apartment, both of which I had when I first asked for a O visa for retirement.

It will be the same to get the extension but you will need a new non-o visa. Or you could do a conversion to a non immigrant visa entry.

Can only guess, that legally staying 5 years in Thailand on extension will by no means be an obstacle on obtaining a new O visa for your next entry.

And in general a Non-O visa is issued without a thought if you fulfill the conditions and have no negative record/visa history.

The 5 year stay is closer to be an advantage in my opinion.

Edited May 1, 201412 yr by KhunBENQ

If anything it would be an advantage, esp. if you've been working with the same immigration office for all those extensions. By this point, you probably speak some Thai, they recognize you, you could explain how/why you missed the previous window for an extension, and your financial position is presumably settled (i.e. long-established pension income or bank account) They understand that people travel or have other reasons for leaving the country and sometimes just have to start the whole process over again.

Many offices will allow you to apply 45 days early for the extension.