Sinbad1, nail on the head, two inspections should be reqd and system should be tested.
.
These oxygen systems fitted to aircraft can have Left and Right thread parts in the system, SOV's can be fitted in any location or orientation, very very easy for things to be fitted wrong.
.
As a general rule, Pilots can re-check each others work/functions, Cabin Crew even cross check doors man/auto modes, Engineers often work alone, work it out.
.
Another point to mention, parts/systems are often fitted wrong for long long periods of time, if they are never needed during service they will not come to light, even during parts/systems maint at a later time, incorrect fitting may not come to light, but the poss of new parts/systems being fitted in the same incorrect fashion can be increased.
