Four days after servicing a B737-900 cockpit Crew O2 bottle; a Mechanic was informed the bottle discharge line to the fuselage overboard connector was found loose.

Narrative

When I was performing a Service Check on a B737-900 aircraft; I found the Crew O2 [pressure] is at 'below normal'. Per the Service Task Card; I serviced it to full. After connecting the 'B' nut and performing a leak check; I discovered a leak at the 'B' nut. I repaired the leak by replacing the union assembly. After installing a new union assembly; the leak disappeared. Then I proceeded to install the discharge fitting and reconnect the overboard discharge line when I got called out to help someone else. When I came back to my work area; I quickly verified the valve on the [Oxygen] bottle that [it] is in an 'Open' position and its safety wired. The airplane needs to be moved right away because it's blocking another airplane that needs to be moved to the gate. I rushed to close the panel and the Ground Power was already disconnected. I used Maintenance Manuals (M/M) 12-15-21 and 35-12-11 for my reference. I got called out by a fellow Technician for help during installation. The B737-900 needed to be moved because it was blocking an airplane that is ready to be moved to its gate. A fellow Technician found the problem while servicing the Crew O2 bottle. If a Technician is interrupted while doing his or her work; he or she must go over the repair process from the beginning of the manual.