What are omens? Omens are normal things which happen in everyday in normal life but could hold special meaning for you / your family. For example, one of the passages mentions that

As soon as Duryodhana was born, he began to cry and bray like an ass. And hearing that sound, the asses, vultures, jackals and crows uttered their respective cries responsively.

Taken individually, these are normal things which happen everyday. Any new baby cries and all the animals make sounds. But in this particular instance, the sages of the time interpreted it as an omen.

This omen pertains only to Duryodhana and his family. As such, most omens pertain only to one particular person / their family.