#onion_omega:matrix.org web app This link will take you to a page allowing selection of a client then send you to the room.
For the oldies of IRC as me, who are used to use text-based chat, just use the second option @David-Godfrey mentioned:
https://riot.im/app/#/room/#onion_omega:matrix.org this is browser based and you can't access as guest. It makes sense to create an account (see top of page) with similarities as you use in the onion community (same name or same personalized avatar).
See you there ...