The capital omega (Ω) is used to show ohm in physics. The letter is known as the Big O in contrast to omicron, which is known as the little O. Omega Symbol in Greek Alphabet (Ohm Symbol) In the Greek number system, it was taken as the symbol of 800. 8 Which is the correct symbol for an ohm?.7 What does the omega sign mean in physics?.4 What does Greek letter Omega look like?.