Is the object of a preposition always an object noun ? Please explain?

Answer 1

That question doesn't make any sense. There's no such thing as an "object noun". There are object pronouns- me, him, her, us, them. A noun can function as a subject, the object of a verb, the object of a preposition, etc. So yes, an object is an object. If you're studying another language, there may be different forms of nouns for objects. That's not true about English. 

Answer 2

No. An "object noun" is the noun receiving the action of a verb, not the object of a preposition. 

If your sentence is, for example, "I took the dog for a walk" then "I" is the subject noun, "dog" is the object noun, "for" is the preposition, and "walk" is the object of the preposition.