The [Mounted Combat][5] rules say...

> A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules..."

This requires that the mount be willing, **but it does not say whether the rider has to be willing.**

Some instances in which a rider might be unwilling could include:

*If the rider had been grappled and carried by a creature that otherwise met the definition of a mount

*If the rider began as willing but the mount was later charmed, polymorphed, frightened, or had some other change of condition such that the rider no longer wished to be a rider but had not yet had their turn and so could not dismount

*If the rider was incapacitated or rendered unconscious and so could no longer give their consent as a willing rider (but by DM's decision could physically remain on the mounted creature)

[Note for the first possibility: The [grappling][2] rules say...]
> When you move, you can drag or carry the grappled creature with you [...]


[Note for the last possibility: the "Mounted Combatant" Feat explicitly requires that the rider not be incapacitated to use the feat, but the Mounted Combat rules do not]

**In any of these cases, would the removal of the rider's willingness to be mounted mean that the rider would no longer be treated as being in mounted combat,** even if they had not yet had the chance to dismount?

**In particular, would an unwilling rider still suffer opportunity attacks if moved by the mount on the mount's turn?**

The [opportunity attacks][1] rule says...

> You can make an opportunity attack when a hostile creature that you
> can see moves out of your reach. [... But you] don't provoke an
> opportunity attack when you teleport or when someone or something
> moves you without using your movement, action, or reaction.

But the [mounted combat][3] rules say...
> [...] if the mount provokes an opportunity attack while you’re on it,
> the attacker can target you or the mount.

  [1]: https://www.dndbeyond.com/sources/basic-rules/combat#OpportunityAttacks
  [2]: https://www.dndbeyond.com/sources/basic-rules/combat#Grappling
  [3]: https://www.dndbeyond.com/sources/basic-rules/combat#ControllingaMount
  [5]: https://www.dndbeyond.com/sources/basic-rules/combat#MountedCombat