B737-800 Technician reported failing to fully open the valve on the crew oxygen bottle and also improperly read the pressure gauge during a system check per the Aircraft Maintenance Manual. Another Technician later discovered the error during an operations check. The discrepancy was addressed and fixed.

Narrative

After servicing the crew oxygen bottle on Aircraft X; I failed to fully open the valve on the bottle. It is my belief that I encountered a resistance partway through turning the knob; and mistakenly believed it to be open fully. I then backed the knob off 1/4-turn and safetied it per the AMM (Aircraft Maintenance Manual). Following this; I proceeded to test the system in the flight compartment. I neglected to properly read the pressure gauge; and in doing so failed to notice that the pressure drop exceeded limits by a considerable margin. The error was discovered and corrected by another Mechanic later on Day 0 at about XA:00 during an operations check. This Mechanic then reached out to me; and informed me of what he had found in a constructive and helpful manner. The condition caused by this error is one that could have caused substantial risk to all onboard in the event of decompression or similar incidents; if it had not been found. I must more carefully read the instructions laid out in the AMM; and take steps to be more positive that I am putting aircraft equipment into the condition intended and necessary.