Originally Posted by
Dash8widget
Generally speaking, under part 91, published ODP's are optional, ATC assigned departures (including headings) aren't. But under 121 or 135 there may be company policies that come into play. My previous company had guidance in their FOM for IFR departures that were generally more restrictive than the FAR/AIM - basically, if we weren't assigned a named SID or given a departure heading (like from an uncontrolled field) then we had to fly the ODP regardless of the current wx conditions.
What he said.
Your companies FOM most likely has guidance on what you are supposed to do.
Something like this:
If IMC or unable to maintain visual terrain clearance and no other ATC departure instruction received, the ODP must be flown. (ATC is taking responsibility for terrain clearance when issuing the vector)
VMC and able to maintain terrain clearance visually it is optional unless company specifically requires it for that airport.