I have bookmarked many pages containing the words pop and merge, but when I search for pop and merge I do not get any hits.

It should return all the pages that have the words pop and merge in the full-text.

I have tried to set match:OR.

It would be great if there was an option to set match:OR as the default search mode and then use quotes for any exceptions.

This is the logical bahaviour and default for any modern search engine.

You can check this site out as an example of a site with the words merge and pop, which doesn't show up in raindrop search: