It will be great if uBlock Origin will have such an option.
because uBO is not Adblock??? Anyways, there's a white-list feature, use that.
What I ask for is black list, to block only specific sites. It can be very useful for many people.
https://github.com/gorhill/uBlock/wiki/Dynamic-filtering:-turn-off-uBlock-everywhere-except
If you specifically want that, install uMatrix and run it in Allow all/Block exceptional mode. That will blacklist only those websites you intend to block.
https://github.com/gorhill/uMatrix#umatrix
https://github.com/gorhill/httpswitchboard/wiki/How-to-use-HTTP-Switchboard:-Two-opposing-views#the-allow-allblock-exceptionally-approach
The solution to turn off ublock "everywhere except.." is good but works only on the elements level and not on on website level regardless of the specific elements.
Most helpful comment
https://github.com/gorhill/uBlock/wiki/Dynamic-filtering:-turn-off-uBlock-everywhere-except