I asked the Thailand consul in Los Angeles if a one way plane ticket was good enough when I arrive with my approved retired o-a visa.

Thier answer... "That depends on that Thai Immigration at the airport."

Not a solid answer.

What does the law say in this regard.. Anyone.. Please.. Cheers.

if you have a visa of any type a return or onward ticket is not required.

Immigration never asks to see a ticket out it is the airlines that will ask if you don't have a visa.

I have an Non-Immigrant "O" with retirement extension and enter Thailand many times every year on One-Way tix and my entry has never been questioned.

Per above, only when you do not hold a visa do you need proof of onward travel.

Check below about boarding visa exempt.

http://www.thaivisa.com/forum/topic/754481-boarding-to-thailand-without-return-ticket/

I just flew from Incheon to Chiang Mai on Korean Air. One way ticket. No questions asked by airline or immigration.