We miss the old web. Don't you?
If you miss useful search results, index pages, chronological feeds, webrings, and personal sites made by humans, then OP is for you.
Build
OP exists to make it as easy as possible to create useful, human networks with regular websites. With OP, you can have hashtags, mentions, and groups anywhere you can host a webpage, just by pasting in one or two lines of HTML (or javascript if you want to be fancy.)
Browse
An OP server is like a combination of a search engine and an old-school web portal or index page. OP doesn't crawl pages, it only shows you what actual humans ask it to. So there's no SEO game to cheat at. And there's no algorithm, because you can make your own feeds.
You can subscribe to most anything on an OP server with an RSS reader.