We had a 737 for a while, on which we were servicing the crew oxygen cylinder everyday. We did leak checks, but couldn't find any leak, until one of us noticed that the F/O regulator was set to "Emergency" and thus slowly bleeding the oxygen system.
On most crew oxygen masks in use today there are three positions, usually named "Emergency", "100%" and "Normal".
In "Emergency" the mask reglator provides a steady stream of oxygen, e.g. for clearing smoke from the mask, in "100%" the mask delivers pure oxygen on demand, e.g. the pilot has to actively inhale to open the demand valve, while in "Normal" the mask mixes pure oxygen on demand with ambient air.
When stowed the mask regulator is supposed to be sent in the center "100%" position to prevent leaking the oxygen supply.
After we informed the crews about the proper operation of the mask regulator, the excessive use of oxygen stopped.