On a failed save, the target obeys the Flee command by
[Spending] its turn moving away from you by the fastest available means.
Does that mean they take the Dash action?
If not, do they take the Disengage action?
In case the answer to 2 is no, do they provoke Opportunity Attack?
I'm leaning towards 'yes' to 1 ('fleeing' seems to suggest 'dashing'); no to 2 ('fleeing' doesn't suggest 'careful retreat'); and 'yes' to the 3rd (since it's not 'forced' movement, at least as per PHB page 195).
If the Dash action is the fastest means of fleeing available to them, then yes, they will use it. For a counterexample, a monster with the ability to teleport (like a yugoloth) would probably do that instead.
If the Disengage action is the fastest means of fleeing available to them, then yes, they will use it. (I really can't think of a way to make this true, though.)
A creature under the effect of a Command to flee uses its own movement, so yes, it will provoke opportunity attacks.
You also don’t provoke an
opportunity attack when you teleport or when someone
or something moves you without using your movement,
action, or reaction (PHB p.195).
The combat effectiveness of the Command spell is always a matter of how the GM interprets:
The spell has no effect [...] if your command is directly harmful to it.
If you explicitly include this proviso to the text of the Flee option, it becomes "The target spends its turn moving away from you by the fastest available means which does not cause it direct harm". If the target has a credible expectation of being stabbed, I'd rule that they Disengage rather than Dash. So:
The target Dashes if they don't believe they'll be exposed to direct harm in doing so, such as opportunity attacks.
The target Disengages if they do believe they'll be exposed to opportunity attacks.
The target provokes opportunity attacks as usual if they did not choose to Disengage. (Which suggests they were mistaken about who was armed and dangerous.)