(from the Quora answer here.)
Hindi readers may have noticed it in Devanagari as ओ३म्. The ३ in the middle refers to the number “3” (as it is written that way in the Devanagari). To explain what it means:
In Hindi (and roughly in every language, except that in most, only two are defined in theory), vowels may have three ways of being spoken, and they are counted as “maatras”. “Maatra” is roughly a measurement of time for which the vowel is uttered. All the vowels are listed here:
अ आ इ ई उ ऊ ऋ ॠ ए ऐ ओ औ
1. The vowels with a single maatra are uttered very briefly (like “e” in “pen” and “u” in “hut”), and they are अ इ उ ऋ ए ओ. (hrasva swar = “briefly uttered” vowels ह्रस्व स्वर)
2. The vowels with a double maatra are uttered for a slightly longer time (like “a” in “art”, and “oo” in “shoot”) These vowels are आ ई ऊ ॠ ऐ औ (deergha swar = “long” vowels दीर्घ स्वर)
Note, that each single-maatra vowel has a double-maatra “partner”, as even in English (e.g. book and boot)
3. Apart from this, there are vowels which have three maatras. These have to be uttered for an even longer time than the “oo” in “shoot” and “ee” in “keep”. These are uncommon vowels and are not in regular use in writing, so they do not have a particular notation. Instead, if you wish to utter a vowel for a very long time (like, say, HEEEEEEYYYYY!! or WHOOOOOOOOO? in English and “arreeyyyy” in Hindi), you need to denote it with the single-maatra vowel followed by the digit “3” (३) in Devanagari.
So every vowel ends up having three forms in Hindi. The ॠ, a dirgha swar, is now archaic and limited to use in Sanskrit. The triple maatra vowels are now limited to ओ३म् (om) and other words in very Sanskrit texts (such as हे३ राम or there are words which use it in the middle, such as ओ३म् itself).
Therefore, it is important that the pronunciation of “om” be made as a very long “o” followed by a very brief “m”.