Quoting you page at

http://www.thaivisa.com/422.0.html

"Your best choice is to obtain a multiple entry Non-Immigrant visa which valid for 12 months. With this visa type you will need to exit and enter Thailand at least every 90 days to get a new entry stamp allowing you to stay a further 90 days.

If you utilize your visa the last time just before it expire, you will get an extra 3 months "bonus", so the total validity will be nearly 15 months."

Is this last paragraph still valid, in particular for "O" visa?

Re-enter the day before the validity expires and you will be fine. Another 90 day stay is yours.