A partial fill to about 200 psi and then a purge to about 50 psi and then a fill to what you want (1,800-2,000 psi) generally takes care of any moisture. If there are particulates in the cylinder, well, that is another problem. It is best to remedy this by getting an actual test and fill at a qualified facility.

And, as an aside to Bob Kuykendall, be careful of the medical design pulse demand delivery systems. They are NOT compensated for altitude. That is where the MH EDS system has the advantage. You require more O2 at 17,999 feet than at sea level, and the (patented) MH EDS is calibrated for that differential. Most of the Continental US is under 5,000 msl (with the vast majority of elevations under 1,500 ft. msl). This is the range for which most medical pulse demand systems are designed.

Depending on a standard medical O2 conserving system may lead you into a dangerous situation while flying at high altitudes. I recommend keeping a reliable pulse oximeter in the cockpit and check your O2 saturation regularly at high altitude.

I logged 142 hours of soaring time last year. I was on 02 for 132 of those hours. It is essential to know your system and its capabilities, as well as your own, while flying at high altitude with a supplemental oxygen system.