You pose an outstanding question! Please allow me to attempt to address it as best I can.
In your first example with OH
: an EV
crew and aircraft could operate an 'extra section' in the place of the OH
flight. However, the OH
crew could NOT operate the EV
operate under separate certificates, and as such, the crews and aircraft of the two airlines are not interchangeable.
In your next example with DL
and 'Song' and United and 'Ted', crews and aircraft could be interchanged as necessary. 'Song' operates under the DL
certificate and is manned with DL
crews (pilots are mainline and can go back and forth, however the flight attendants are singularly with either mainline or 'Song' and are not interchangeable. Likewise, 'Ted' operates under the UA
certificate (however, I am not familiar with the crew workings for the two).
In your last example: yes, as above with EV
, an extra section could be flown by the second carrier which has the additional aircraft, but the crew with original carrier could not man the second company's aircraft.
Ignorantia juris neminem excusat.