Just trying to work out the Mechanics. So if you where 5 level and had both booming Blade and Hex Spell, does the Hex hit 3 times here?

1-You hit opponent with sword and booming blade, 1d8 is added for booming blade and 1d6 is added for Hex

2-They move, taking a further 2d8 from booming blade + 1d6 for hex? and gives an AoO

3-You hit them again (can't use booming blade again for AoO) but you still get the 1d6 for Hex right?

So that can be 3 Hex in 1 turn?

Looking at the Hex Spell this line is key I feel

"  Until the spell ends, you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack "

Technically you only hit it with an attack twice (original hit with Booming Blade & AoO) so I would rule (in my game) that Hex is only applied twice

Thanks, yes that makes sense as the 'move' damage did not require an attack roll.

If you roll a d20 to determinate if your attack hits, you can apply Hex. 

Note, if you have the Warcaster feat, you COULD use Booming Blade for the  AoO.

That seems beside the point of this discussion about Booming Blade and Hex.