I usually put my boss in my OOO, because if something is so urgent that it needs to be delegated RIGHT NOW then it’s urgent enough that my boss should know about it, and he’s also in the best position to know who on the team to delegate it to based on everyone’s workloads and what can be dropped. But the most likely result is that whoever is emailing me either waits for me to get back because it’s not that urgent or goes to the next/backup person based on our central documentation about who to contact for particular issues.
Protocol for when you should use an out-of-office message will vary by job and industry, but generally, you should set an OOO message when you’ll be out for two or more days.

Inform your correspondent about the date by which they can expect to receive a response to the email they send you during the holiday season. Indication that your will reply to the email when they return.
If you are going out of office but still leave something interesting for your contacts like poetry, that’s something really out of the box. This is a truly amazing OOO message, and reverting with poetry will surely make the receiver read it twice as emails are the last place one imagines reading poetries. Especially closing with “If all else fails, clear your cache…” is one good way of telling people that they will need proper help and it will anyway take time before they’re all set. Have a look at it here: