If you can get naloxone, it's the nasal spray and easier to administer. The injection stuff lasts longer, and it's also easy to administer, but requires a step more.
Training is required, but quick and online, includes a few things on how to recognize an OD, ways to wake someone up (chest rakes), and how to administer.
Def do carry it in something like a work bag, you never know who you can help.