OOAK stands for One of a Kind.
These are yarns which in all probablility won't be repeated. When designing new colourways there are always trial dyeing sessions where you don't quite get the colour or combination of colours you are looking for. And so the OOAK hanks are created. There is absolutely nothing wrong with them - they are simply unique and won't be repeated.
They are offered at a slightly reduced price only because they are OOAK and cannot be replicated.