Pronunciation of 壹 / 一 in Mandarin The pronunciation of 壹 / 一 in Mandarin, when I was in school, is:

yī


It's been a while that it is also pronounced as:

yao (or similar, I'm not very good with pinyin)  


and seems common among native speakers. Is it a slang or official and is there a reason?
「壹」or「一」is not pronounced yāo; this is an alternative word for the number one in certain circumstances, and is the most common spoken representation of one when talking about phone numbers.

The character for yāo is「幺」, which is the top part of the silk「糸」component. It originally represented a small amount of silk thread, extended to mean a small amount (cf. 幺麼 tiny/minute and 老幺 youngest child of a family), and further extended to be an alternative representation of the number one.