Typing ओं is currently not possible in hindi transliteration due to a rule-conflict.
Typing ओं requires input of oM. However, due to the rule
['ओM', '', 'ॐ']
This combination is reserved for ॐ, making writing ओं impossible.
This is an unfortunate side-effect, but needs to be resolved somehow, and quickly.
I say quickly because several words in hindi, when used in plural form in a sentence, end in ओं . Examples: भाषाओं, चिताओं, कलाओं, घटाओं etc.
I don't know why this bug wasn't noticed earlier, but it needs to be resolved somehow ASAP.