Copper for inline O2

I'd like to build a connection from my inline O2 check valve to my flow meter out of rigid copper pipe. I need something to support the flow valve in a vertical position.

Since the inlets are on the back of the flow meter, I can't mount them to the wall as planned. Brass seems like a lot of fittings and what not. So basically, will copper be okay for O2?

Thanks.

I have read two interesting things on this so far.

1) Copper is fine for low pressure air lines as long as the pipe is cleaned and does not contain any residual oils.

Why cleaned?

2) If oxygen hits oils at any kind of pressure it is likely to ignite.

So my next question is, how can I assure myself there are no oils in my copper? Is a rinse with oxyclean enough? Does Brass not have this problem?