oculus

oculus, oculi is a second declension masculine noun, regular throughout, and it is the plural oculi, oculos, oculis that you meet most often. "With my own eyes" is the bare ablative, meis oculis, with no preposition: Latin treats the eyes as the instrument.

The word carries a diminutive suffix that has stopped meaning anything, and it comes from the same ancient root as Greek optikos and English "eye". Latin then built another diminutive on top, ocellus, "little eye", used as a term of endearment. Descendants: Italian occhio, Spanish ojo, French oeil. English gets "ocular", "monocle" and, unexpectedly, "inoculate", which was a gardener's word first, for grafting in an eye or bud.