Is OM an imported religious term from Hinduism? OM or AUM is used in "OM MANI PADME HUM" and some other mantras.

To my knowledge, OM is found only in  Mahayana and Vajrayana schools. It's not found in Theravada. The 14th Dalai Lama explained the meaning of OM here: On the meaning of OM MANI PADME HUM.

It's an extremely important and venerated word in Hinduism -- see "What is the significance of ॐ (Om/Aum)?" on Hinduism.SE.

Is OM in Buddhism an imported religious term from Hinduism?

Or like "namo" which means "salutations" or "adoration", does OM have an ordinary meaning and therefore not considered an imported religious term?
OM is basically associated with Shaivya tradition of Hinduism i.e. those hindus who worship Shiva. And according to hindu mythology Shiva's abode is Kailash mountain which is in tibet. So inhabitants of that part of Tibet were using the word AUM or OM either for purpose of meditation or as ordinary word to  mean Consciousness. Hinduism was mostly rooted in Afaganistan, Pakistan and the Gangaitic plains of India during ancient time. 

With this line of reasoning there is a heavy possibility that Hindus themselves adopted OM from Tibeteans. 

OM is not used by Theravadins only by Vajrayana Buddhists and Mahayana Buddhist whose roots are in Tibet. So basically Tibetians are using it and it belongs to them. 

So I wont say it is imported.