"op" is Arc's idiom for web request type, so that attribute makes it easy for a scraper to tell what kind of page it's looking at. We added it for a Chrome extension we wrote to help with moderation.
One of these years I want to open-source a stripped-down version of that Chrome extension, or better still make it an optional JS client that people can opt into. It's handy for browsing HN, if you like keyboard shortcuts and don't mind spawning tabs. Anybody know if there's a way for regular JS to spawn tabs, if the user is willing to give it the right permissions?