Paramedics and EMTs!
Here's a simple little program that allows you to specify oxygen cylinder size, tank pressure, and flow rate to calculate how long the tank will last. There's a "set alarm" button which allows you to exit the application and do other things, then receive a reminder.
Quick Pick option for nasal cannula, face mask, and non-rebreather, or override with your own LPM specification. Also allows you to enter vent settings including tidal volume and FiO2%.
Full explanation, including formulas and limitations, in the program help (Menu -> Help).
Completely free, and ad-free.