I absolutely believe that there is no contradiction between Hindu theology and modern science. Rather, in many fields both of them meet logically. Cosmogony (the study of the universe) is such an area where the matter of Om and the Big Bang meet clearly. Actually two processes are indifferent but we look at them through two different windows.
Hindu theology says that Om is the primeval sound from which the entire universe was created. Om denotes the Param (supreme) Brahma. But as we know God is threefold in nature, same is the case for the single syllable Om. It consists of three different letters-A, U and M which are the seed of all other letters, sounds and words. A, U and M can also be signified as Brahma, Vishnu and Shiva or creation, preservation and annihilation or satva (goodness), rajas (passion) and tamas (ignorance). In the Mandukya Upanishad, the Taittirya Upanishad, the Bhagavat Gita and many other Hindu texts deal with the matter of Om.
According to the Big Bang theory, our universe is thought to have begun as an infinitely small, infinitely hot, infinitely dense, something- a singularity. After its initial appearance, it apparently inflated (the Big Bang), expanded and cooled going from very small, very hot to the size and temperature of our current universe. Now, we can understand clearly that Om and the Big Bang are almost the same. The singularity mentioned in the Big Bang theory is nothing but the Param (supreme) Brahma who uttered the sound word (Om) and the creation of the universe began. But there is a little difference between Om and the Big Bang. Om was actually a sound word where as the Big Bang was only an expansion (not explosion). I think there is a flaw in the Big Bang theory. The theory says that prior to the moment of the Big Bang there was nothing. But I think, during and after that moment there was something. I also think, something surely comes from something and Sanatan Hindu theology also says so. We, the Hindu devotees, strongly believe that He (Param Brahma) is the Creator of the entire universe and all the creatures. He is the past, present and future of all. He is sanatan. He is shasvata (eternal).
Thus, the existence of Supreme Creator before Big Bang is proved in Hinduism and this is the little difference between Om and the Big Bang theory.