Because the tissue inside the tooth, i.e. the pulp, reacts badly to the drilling stimulus (heat and vibration), it frequently becomes severely inflamed and precipitates a condition called acute or chronic pulpitis.
This condition usually leads to severe chronic tooth sensitivity or actual toothache and can then only be treated with the removal of the nerve (pulp) called root canal therapy.
But, ZOE had in vitro cytotoxicity majorly due to release of Zn ions, not eugenol.
The vegetable or mineral oil acts as a plasticizer and helps to counteract the irritant action of eugenol.
Rosin added to the paste in the second tube speeds up the reaction and produces a smoother, more homogeneous product.