Letting karlash consume as much as a single soul coin is definitely a oath break if i ever saw one

[General Nuclear]

Playing as a paladin and letting karlash consumed a soul coin is the very epiphany of breaking our oath.

Karlash is wrong, As long as you got a scrap left of the soul in any shape or form then you can reforge it from what i have heard, Not sure if it was D&D or pathfinder, but i have heard that you can even frankenstein fragments together to make a new soul as well.

Contract or no contract, letting a helpless soul get consumed for the sake of power is the least paladin thing to do.

[hannibal_pjv]

Depends on you oath… But yeah it should break some of those oaths.

[Tenacity]

Oath of Devotion? Most likely an oathbreak, yes.
Oath of the Ancients? 100% absolutely an oathbreak, no questions asked.
Oath of Vengeance? This is completely standard OoV behaviour. Almost never an oathbreak, unless using the soul coins would empower the foe you have sworn the oath against.