Gregory's Server
1. You can search for a hashtag, e.g. #OpenSource on the search interface (web)
or
Change the URL https://mastodon.online/tags/OpenSource with a hashtag of your choice.
2. If you are on the web interface, you can see the search results and an icon (person) next to "Back". Click it to follow the hashtag.
3. Now toots related to OpenSource will appear on your timeline.