I sent an encrypted email. It had me choose a one time password for the recipient to use to access the email. How do I inform the recipient of the one time password in a secure way?
It depends on your use case and the relationship with the recipient. You could call it to them - that's about the best.
For less sensitive things, you could send it over some other medium such as text it to them, skype it, etc.
WhatsApp or Signal are great apps to use because they are encrypted.
I'm working on a system that will not require you to communicate that password, and hope to release that sometime in the future.
Warning: please do not send one-time passwords over email.