Because the Non Imm O visa only gives you 90 days, they don't know you're applying for an extention of stay and if you do apply its a different Government Department, ie extentions are from RTP and visas from Embassy/Consulate are MFA.
The financial requirements for the 1 year extention of stay are 40,000bt per month or 400,000bt, there's no financial requirement for the 60 days extention of stay.
You certainly can get the 90 days Non Imm O outside the Country, it's available at the RTE Washington DC and just about every Thai Embassy/Consulate. You CANT get the 1 year extention of stay without first having a Non Immigrant O or OA. Most people go for the O as it doesnt require insurance for the extention. STOP confusing yourself and other members!
You've got that the wrong way around. You can only get the OA in your home Country. You can also get the O in your home Country and at pretty much any Consulate/Embassy