This plugin doesnt work (possible due to manifest V2 being deprecated? no clue.) but opera already natively allows you to do this (as I found out from a helpful reddit comment)

Go
to the URL "opera://flags/#sidebar-site-panel", enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to Custom Site Panels, and add the specific Reddit URL you want. You can do this with other sites too.

Note though that custom site panels are strict. Redirects to other domains won't work and click links to other domains will open them in tabs instead of in that sidebar panel. In the case of a site like outlook.com that redirects to live.com for logging in, you'll have to do that in a regular tab first to get cookies set before things outlook.com will load right in the site panel.

Also note that you can't customize the site panel's icon in the sidebar.

But, have fun with it. - https://www.reddit.com/r/OperaGX/comments/113m6jq/comment/j8wgc6w/

This plugin doesnt work (possible due to manifest V2 being deprecated? no clue.) but opera already natively allows you to do this (as I found out from a helpful reddit comment)

Go
to the URL "opera://flags/#sidebar-site-panel", enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to Custom Site Panels, and add the specific Reddit URL you want. You can do this with other sites too.

Note though that custom site panels are strict. Redirects to other domains won't work and click links to other domains will open them in tabs instead of in that sidebar panel. In the case of a site like outlook.com that redirects to live.com for logging in, you'll have to do that in a regular tab first to get cookies set before things outlook.com will load right in the site panel.

Also note that you can't customize the site panel's icon in the sidebar.

But, have fun with it. - https://www.reddit.com/r/OperaGX/comments/113m6jq/comment/j8wgc6w/