The best vessel would be an aluminum scuba tank. The oxygen bottle is probably steel - a poor choice. Steel cylinders have thinner walls than aluminum and are very susceptible to corrosion. A typical 80CF @ 3000 PSI scuba cylinder will have a wall thickness of of something like 3/8" and are very corrosion resistant. Furthermore, aluminum oxide actually inhibits further corrosion while rust on on teel tank begats more rust. It's best to use a bare cylinder. All paint does is trap moisture underneath - encouraging corrosion.

I''ve worked full-time in the recreational dive industry for 12 years by the way and I can't BELIEVE I didn't think of using a scuba tank!