At what point in a creature's movement does an opportunity attack take place?

Question

The specific situation is as follows:

My warlock with War Caster has a bonfire from Create Bonfire the tile in front of them, and a hostile creature in the bonfire. On my warlock's turn, I successfully affect the hostile creature with Dissonant Whispers, provoking an opportunity attack through movement, satisfying War Caster's requirement.

Can I cast Lightning Lure, as my opportunity attack, once the creature has used 15 feet of movement, in order to attempt to proc damage from the creature leaving (upon being forced to move by Dissonant Whispers) and re-entering (upon being pulled by Lightning Lure) the tile with the bonfire?

Or, must I take my opportunity attack immediately upon its trigger, while the creature is still effectively 5 feet away?

Answer 1

It must be taken immediately

When a reaction is taken, it immediately follows the trigger (emphasis mine):

Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind

The trigger for the opportunity attack is...

when a hostile creature that you can see moves out of your reach

... so as soon as the creature leaves your reach (typically 5 feet), you must make the attack.

Note: In general, reactions occur after their triggers, but opportunity attacks make an exception:

The attack occurs right before the creature leaves your reach.

Answer 2

Assuming your reach is 5ft, then your opportunity attack is executed before the creature leaves the fire tile. According to the PHB (pg 195):

The [opportunity] attack interrupts the provoking creature's movement, occurring right before the creature leaves your reach.

Opportunity attacks are fundamentally about melee strikes, so the range is based on your melee range (ibid):

You can make an opportunity attack when a hostile creature ... moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack ...

If you have a class feature or similar that allows spells to be used for OAs, unless the text were to override the "reach" requirement explicitly, you would follow the general rule as above.