EDIT The Ohm Sign is the capital Greek Omega. In electrotechnics, it is the sign for the electrical resistance (the ratio of voltage to current).

The Omega is the 24th and final character of the Greek Alphabet. In conjunction with the letter Alpha it is often used to symbolize the beginning and the end.

 jsw 

EDIT Unicode logic demands the existance of a separate codepoint for the mathematical character Ohm (U+2126) and the Greek Capital Letter Omega (U+03A4) although they are the same character.

 jsw 

EDIT Type Design The mathematical character Ohm (U+2126) and the Greek Capital Letter Omega (U+03A4) are the same character, but they can have different glyphs if so desired. The Adobe fonts for example use two glyphs. In every case it is important that the type designer positions a glyph at both codepoints.

 jsw