Different
I think that CO2 and O2 has different threads, they really don't want people to get them mixed up, "Why's grandpa that funny color, I just got him a new bottle of oygen..."
Try several onlline retailers, I boght mine from Aquabotanic, but very nearly got it from Aquatic-Store, both have good reputations. Also customaquatic, glass-gardens, there are several choices that I've heard of.
I have a Milwauke All-in-One and a JBJ All-in-One. Add CO2 proof line and maybe a check valve with the order, and your choice of reactor (there are so many) and get a 5 or 10 or 20 lb tank locally, I got mine full at a welding shop nearby. Be sure to get the plastic washer and tighten it up really tight, no teflon tape. Then you are good to go.
You can piece it together, but I've heard of too many guys who drove all over trying to find some certain connection to fit A to B. If you buy as much as possible at one place, they can help you get what you need the first time.