that's where you confused me. A SID is a workload reducing ATC clearance, but a clearance. An ODP may be part of a clearance but in the absence of a clearance, a pilot prerogative (at least in the absence of a SID, which also gives obstacle clearance).This has been covered in previous threads. Others can probably explain it better, but I'll take a stab at it. The requirement to fly an ODP or an ATC-assigned DP is in 14 CFR 91.175(f)(3). Note that the beginning of paragraph (f) states "This paragraph applies to persons operating an aircraft under part 121, 125, 129, or 135 of this chapter." No mention of Part 91.
AIM 5-2-8 says that Part 91 pilots are "strongly encouraged" to file and fly a DP, but that is not synonymous with "are required to."
By the way, the same AIM paragraph says that "ATC clearance must be received prior to flying a SID," so I was mistaken to include SIDs in my statement.