You are getting an Extension of Stay based on being age 50 or over... related to the 90 day O visa you have. Not a new 'retirement visa'.

You can apply by mail

These medical certificates ask / test only for communicable diseases.

There is a lot of misunderstandings on the part of those asking 'How To'? Questions

You can get a retirement visa in Australia without being retired - only have to be age 50 or over. Then have the Equivalent of 800,000 Baht in an Aussie Bank, plus a medical certification and criminal background check -both actually easy and simple. You go to a Thai Embassy / Consulate in Australia (not Thailand) and get a Non Immigrant O-A Visa

Should you choose to - you could first fly to Ho Chi Minh City, Vietnam - go to the Thai Consulate there and get a 60 day tourist visa which can be Extended 30 days at Thai Immigration inside Thailand. Or even stop first in Hong Kong.

It is normal under the O-A visa (in the first year) to be stamped back in with a date one year out from the date reentered. Managed correctly the O-A visa will yield two years (As it is supposed to do).

In the second year the O-A visa is no longer a Multi-Entry visa. The second year of the O-A visa requires a reentry permit if you exit Thailand in order to keep the remaining portion of the second year alive upon return.

The discussion here in this thread is a special case of reentering after the expiry date of the first year. Read the various narratives above.

My friend did and Immigration said it would work for her O-A

That is what I thought I said. I just left out the word extension... an oversight

My friend who has an O-A Visa just did this - she made a trip to Singapore a few days ago and is back. Yesterday she went to Immigration to report after being out of Thailand. She asked - When I go to Germany in June - I will not return until the first year expires. If I buy a Reentry Permit before I leave in June will my O-A still be intact until April 2019? - Answer - Yes.