If this plugin eventually makes it into WordPress core, I want it to be a 1:1 copy/paste. This means we should better adhere to the core coding standards.

For example, WordPress has an WP_Embed class. How would WP_oEmbed_Plugin be called?
We now have a WP_oEmbed_Frontend class which outputs the iframe content and filters the oEmbed result. Two things that should be separate IMHO.

Should more methods be normal functions like get_oembed_endpoint_url?