Can I apply for a Non-Immigrant O-A visa and enter Thailand months later?

It seems you, as many do, may be confusing two different dates. There is the visa validity date and the permission to stay date (entry stamp). The visa simply allows you to enter Thailand and as long as you enter before the expiration date you will be stamped in for whatever timeframe that visa grants. Or as in the OA visa it will be that or until your insurance expires, whichever is less.

You do not get more than the 2 years on an OA. That 2nd year isn’t your OA visa but a 1 year permission to stay stamp. After the 1st year of your OA you must purchase re-entry permits to keep the permission to stay stamp alive if you choose to travel outside Thailand during that 2nd year. After the 2nd year you can return to your home country to obtain a new OA visa and start the process over again. After the first year you can also just begin applying for 1 year extensions of the OA visa at the Immigration Department inside Thailand. You will have to meet the financial requirements for a 1 year extension and you will continue to need to meet the health insurance requirement.

I had an OA for 6 years. It was a good visa in 2017 but these days I would recommend getting a 90-day non-O visa and just go directly to 1 year extensions. You can still get health insurance but there is no reason to have it connected to your visa.

Yes, your Non-OA will be valid for one year after application, so you have enough time to enter the country. The insurance must be for one year, beginning from the day of arrival, otherwise the will give you a shorter "permission of stay".

I'm not sure what you mean by "waiting period".

A single entry non-O visa is valid for 90 days that start the day it's issued. That means within those 90 days you must entee Thailand to use the visa. Once you enter Thailand you'll be stamped in for 90 days permission to stay in Thailand. Even if you wait until the last day written on the visa which is day 90, you'll still get 90 days permission to stay in Thailand.

A non-OA is a 1 year multiple entry visa. It starts the day it's issued by the embassy just like every visa. During this 1 year you may enter Thailand an unlimited number of times, and every single time you enter Thailand you will receive a 1 year permission to stay stamp (or until the end of your insurance if that's less than a year).

If you apply for the visa multiple months before you plan on traveling you're wasting a lot of time that the OA visa offers. Most people use the OA to get 2 years in Thailand.

Get visa and go to Thailand and receive 1 year stamp. Just before the end date on the visa you purchase a new year of insurance, leave and return to Thailand, and get stamped in for another year. (remember every time you enter during the validity of the visa you are stamped in for a year).

So if you get the visa and then wait 6 months you're effectively throwing away 6 months you could have been in Thailand.