Are there any difference between OVA and Ovum?when i call ova and when ovum?

Ovum is simply the female gamete. Since the nucleus of this reproductive cell contains only a half number of chromosomes as a usual cell does have, an ovum is considered as a haploid cell. Whereas egg is the fertilised state of an ovum with the genetic materials of a male gamete. In fact, an egg could be defined as the organic vessel that facilitates the embryonic development for a zygote. For an ovum to become an egg, the transfer of the genetic material should have to take place.

Ova is plural of Ovum

ovum (Latin: "egg", plural:ova)