Oxygen Cylinder in Vertical Position

That keeps the liquid portion at the bottom. Only gas comes out the top/outlet.

Also I suppose the valve is less likely to be accidentally damaged (or worse, sheared off) if the cylinder is kept upright. If the cylinder is horizontal and the valve comes off what you have then is a missile.

Then it would be best to secure the cylinder in the vertical position, then, and take steps to prevent its falling over and breaking the valve off, otherwise someone is going to get very cross.

Would some kind reader please find the video clip of those hydrogen nutters chopping the valve of a cylinder clean off to watch the thing blast off across the desert, and post a link? Only in America....

MythBusters: Air Cylinder

Not exactly in the desert, but, you'll get the idea.

Unless it is a LOX cylinder, there is no liquid in the cylinder. A conventional medical or welding O2 bottle can be oriented in any direction. HOWEVER an acetylene bottle CANNOT be oriented in any direction but vertical with the valve at the top. the reason is that acetylene is unstable as a compressed gas and will spontaneously explode if compressed in a large volume. Acetylene cylinders are filled with an aggregate and then filled with liquid acetone which the acetylene then diffuses into the acetone.

Do remember that if the valve of the cylinder is broken off you have a rocket flying about that can kill someone or do a LOT of damage.