I am currently in the first year of a Non-Immigrant  O-A Visa, based on Retirement.  I understand that if I leave Thailand and re-enter during the first 12 months I will be re-admitted for a further 12 months from the date of re-entry, thereby avoiding the need to apply for an extension at the end of the first 12 months. My question is: does it make any difference to the extension period whether I re-enter at a border crossing (from Laos) or by air at DMK?   Advice would be appreciated.

You must leave and re-enter before the visa expiry date (enter before) which is not the same thing as the "first 12 months".  But provided you do that you will indeed get a new 12 month entry whether by land or air.

You can do the entries by land or air and get a new one year entry up to the enter before date on your visa. After that date you will need a re-entry permit to keep the last entry valid if you want to travel.

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8 hours ago, falkirk said:

My question is: does it make any difference to the extension period whether I re-enter at a border crossing (from Laos) or by air at DMK?   A

You aren't extending anything. To maximise the benefits of this visa you should enter on, or as close to, the 'enter before' date printed in the visa. Regardless of when you enter, before the 'enter before' date, you'll get permission to stay for 1 year whether entering by air or land.