Omicron ( uppercase\/lowercase Î Î ) , is the letter of the Greek alphabet , used to represent the `` o '' sound in Ancient and Modern Greek . Omicron ( uppercase Î , lowercase Î , literally `` small o '' : Î Î 1\/4 Î Î Ï Î Î 1\/2 , o mikron , micron meaning ` small ' in contrast to omega ) is the 15th letter of the Greek alphabet .