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There's no quick and easy way to convert existing pages (using a pagebuilder or theme) to Oxygen Builder. The best way to make the move would be rebuilding the entire site -- which is actually a blessing in disguise.
When you rebuild from scratch, even if you're recreating an existing design, you can create for performance -- the main reason why people move to Oxygen in the first place.
There's also a helpful tool that'll convert any html (WordPress based or otherwise) called Nimbufy. It's a Chrome Plugin which will recreate the structure and CSS in Oxygen. It works pretty well for rough recreations of pages in Oxygen, but they'll need some work to be ready for production, be responsive, and look good.
James has a guide on Isotropic, showing how Nimbufy works as well: https://isotropic.co/using-nimbufy-to-migrate-any-site-to-oxygen/