Both routes lead to a one-year stay, but the paperwork is very different. The O-A is issued by the Thai embassy through the e-Visa portal and requires health insurance of at least US$100,000, a police clearance certificate and a medical certificate.
The Non-O is applied for inside Thailand. There is no insurance requirement, no police certificate and no medical certificate — but you need a Thai bank account holding ฿800,000 for two months before the first extension, and three months afterwards.
German pensioners using the income route should know that the German embassy in Bangkok no longer issues income affidavits. In practice this means most applicants use the ฿800,000 deposit, or a combination of deposit and monthly transfers.
Plan the timing carefully: money transferred from Germany must arrive as a foreign transfer and be visible in the bankbook. We recommend arriving at least three months before you intend to file.
Whichever route you choose, budget for 90-day reporting and a re-entry permit before any trip abroad.