A First Officer describes the chain of events that led to mechanics servicing the crew oxygen bottle for the Saab 340B with passengers onboard.

Narrative

During preflight acceptance flow; First Officer noticed that our oxygen level was at 1;150 PSI. It was too low and since we were at a hub station per the Flight Manual the Captain wrote the discrepancy in the Maintenance Logbook and was trying to notify Dispatch and Maintenance Control. While getting a hold of Maintenance Control and reporting the Logbook discrepancy number; 2 mechanics overheard us on the radio and came right up to the flight deck and asked what we needed. The Captain told them that our oxygen was low and needed to be serviced. This is all happening while the Captain is still trying to transmit the information over the radio. Another Mechanic in a different vehicle caught our attention and after the Captain left the flight deck; he informed him that you cannot service the oxygen with passengers on board. By the time he got our attention; the servicing was already complete; and we were unaware that it had even started. Both mechanics that performed the servicing were aware that passengers were on board since they came up to see what we needed. I thanked the other Mechanic for getting our attention to the issue but he wanted to argue that it wasn't authorized. We understood that; but had no idea that the servicing had already begun. At that time it was all after the fact; passengers were boarded; and there was no reason to continue the discussion further. By the time I spoke with him; the 2 mechanics had the oxygen level up to 1;600 PSI. The remaining passengers boarded while the Maintenance Logbook was completed and the flight left on time. Callback conversation with Reporter revealed the following information: Reporter stated he believes the crew oxygen bottle is serviced from an external source that connects to the aircraft. He did not know if the crew bottles were made of steel only or a composite type material.