It depends on your tank's tolerances. Some may fit, but be too thin, or too thick, some O-rings are made of different materials. I find it a bit daunting to keep track of mine, so I just put them back in the packaging, so that I know what I have.
But, if the seal is holding and you are getting a good vape from it, you should have no problems. Glad it's working out, since the tank isn't leaking. It can be a challenge at times finding the right O-ring and I've been fortunate to not have gone through very many although I do try to keep the spares organized, I would not be looking forward to a "hunt for O-rings" on fasttech, or anywhere else.
Anna