There is a cow and an ox in the field?

It's grammatically correct, but it doesn't tell us much! To make it more interesting, you could add some details:

* What are they doing? Are they grazing? Fighting? Just standing there?

* What kind of field is it? Is it a meadow, a pasture, or a farm?

* What kind of cow and ox are they? Are they brown, black, spotted? Are they big or small?

For example, you could say:

* "A brown cow and a white ox graze peacefully in the green meadow."

* "A massive ox and a small, nervous cow stand staring at each other in the dusty field."

* "The farmer's prize ox and a mischievous young cow were locked in a playful battle in the field."

Let your imagination run wild! 😊