Can I fill the reservoir a few hours ahead of time?

We do not recommend filling the reservoir in advance.  The reservoir is designed to bleed off some oxygen over time.  This allows an escape path if any moisture were to get in the reservoir.  If you fill long in advance, you will find the reservoir be partially empty. 

A better solution is to automate the filling with an outlet timer that will turn the oxygen concentrator on and fill it right before use.