The American Airworks site tells you that the Sierra Precision booster is not compatible with Oxygen unless a special service kit is installed.
Call them and tell them you want to order the O2 kit.
Electric and Pneumatic Gas Boosters , Sierra Air Boosters and Haskel Gas Boosters for Scuba, Fire and Industrial Applications - 800-523-7222
I called American Airworks, because I swear a few years ago I saw a note that they sold a service kit. It's not on their site anymore. I called and they said they will upgrade them for oxygen service, but you have to send it in.