Om 'ओ३म्'

'Om' is the first or main sound as per Vedic belief . Many Vedic (of Vedas) mantras/slokas (verse/hymn) start with this sound/word.
Using DevaNagari alphabet this sound/word can be written as 'ओम', 'ओम्' or precisely as 'ओ३म्'. There is a specific symbol 'ॐ' (a kind of shortcut) to represent this sound in DevaNagari and other Indian scripts. 'ॐ' is the main sacred symbol of Hinduism. In Hindi 'Dharma' means religion. So it is mainly a religious symbol ('Dharmic Chihna').
As per the 'Vyakaran' (Hindi Grammar) the vowel 'o' is sometimes substituted for two vowels 'a' and 'u', so people also spell 'Aum' instead of 'Om'!
'Gayatri Mantra' like many other Vedic slokas starts with 'OM'. That's why we can safely assume that 'ओम' means 'हे परमेश्वर' ('My Lord' or 'Oh, our God').
Gayatri Mantra (tracts by S. L. Sharma)
गायत्री महामंत्र
ओ३म् Om भूर्भुवः॒ bhoorbhuva(h) स्वः॒ sva(h)।
तत्स॑वितुर्वरे॑ण्यम् tatsviturvarenyam भ॒र्गो॑ bhargo
दे॒वस्य॑ devasya धीमहि dheemahi।
धियो॒ dhiyo(h) यो yo नः॑ na(h) प्रचो॒दया॑त् prachodayaat॥