When you say that someone is `odious’, you are implying that the person is extremely unpleasant. The individual deserves our contempt and hatred. The word is normally used in formal contexts, and can be used with things as well.
*Ritu is certainly the most odious teenager that I’ve ever met.
The word `odorous’ is related to `odour’, meaning `smell’. When you say that something is `odorous’ what you mean is that it has a strong and distinctive smell. The smell can be either pleasant or unpleasant.