Can I extend my 90-day Non-Immigrant O Visa for 1 year and travel back to the UK multiple times without losing my extension?

Extensions do not have any entries. If you want to keep your extension valid when you leave you would need to purchase a re-entry permit which will allow your extension to stay alive when you leave. They are 1000 baht for a single re-entry and 3800 baht for multiple. They do not change the dates of your extension at all, so you would need to be back and apply for another extension before the end of your extension.