I maintain a few small open source projects around publishing math on the web. I have a Freenode IRC channel and Matrix room bridged together where anyone is welcome to join anytime they need help. When I am relatively free, I also post a live web meeting URL to the channel where anyone can drop in or drop out anytime. So the Matrix/IRC channel along with occasional web meetings have been my version of office hours so far.
A wonderful side effect of maintaining a channel like this has been that there is a tiny group of regulars now in the channel and we often discuss stuff other than open source projects too. Due to the nature of the projects, they attract like-minded people into the channel, so a lot of time is spent in discussing mathematics and computer science literature, and Lisp, Emacs, etc. What started as a support channel has gradually evolved into a tiny book club for mathematics and computer science!