[dedward]

Which is kind of weird, considering that the Ohm symbol is by definition the greek Omega - why did we use two separate unicode slots for them?

[billswift]

I suspect they incorporated two different pre-existing symbol sets - the Greek alphabet and a set of scientific symbols.

Added - they did; here is an older version (3.0) of the standard online http://www.unicode.org/book/u2.html