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It's a 'universal' symbol used not only by Hindus but also Jains, Sikhs, Buddhists and others...but how each uses it differs in meaning and practice.

In general Mahayana and Vajrayana (as found in its Tibetan, Japanese and the re-transmitted Chinese Tang Mi forms) Buddhism, amongst the many meanings and teachings on what 'Om' is, it's just one of the many Sanskrit syllables used to represent the body, speech and mind of a Buddha in sound and generally, 'Om' is regarded as representing a Buddha's mind in sound. It can be used as a seed syllable standalone or in conjunction with other syllables.

His Holiness the 14th Dalai Lama, in his teaching on 'Om':
http://www.sacred-texts.com/bud/tib/omph.htm
The first, OM, is composed of three pure letters, A, U, and M.
These symbolize the practitioner's impure body, speech, and mind; they also symbolize the pure exalted body, speech and mind of a Buddha. Another opinion:
The Seed Syllable Om
Oṃ seems not to be used by Buddhists until the middle Tantric period - it does not appear in early Mahāyāna dhāraṇī nor in the Mahāvairocana Abhisaṃbodhi Tantra (ca mid 7th century). In the latter mantras begin "namaḥ samanta buddhānaṃ".
However it becomes the standard way to begin a mantra at some point after this. Buddhist mantras always seem to use oṃ rather than auṃ. Although Tibetan exegesis gives oṃ a variety of different associations and significations, the main function of oṃ in Buddhist mantras seems to be to mark what follows as a mantra. According to Lama Govinda oṃ presents Buddhahood in potential, and is contrasted with hūṃ which represents Buddhahood made manifest. Before the collapse of the E-Sangha Forum, there was a thread topic which discussed a Sutra where the Buddha was reportedly to have taught on this syllable 'Om'. On another forum, I sought to recover this link but was only given a Chinese Sutra link which titles 'Dhāraṇī Sutra for Safeguarding Nation, Realm, and Ruler' where "Om" is explained as a combination of "a", "u", and "m" which signify the Trikaya of a Buddha (Dharma, Reward and Emanation Bodies)

In Thailand, although the majority of Buddhists there are Theravada, there are many verse invocations or 'kathas' for various protective verses, amulets and occult practices which adopts the 'Om' as its beginning or in the middle, some sourced from the local animistic/Hindu practices and some mixed with verses from the Pali Suttas. Just google 'thai katha' and you will find out. See this sample