When a unit of measurement is named after someone, convention has it that the word is written entirely in lower case when talking about that unit, but the single letter abbreviation of that unit is written in upper case.
The unit of voltage is the volt, named after Volta, and abbreviated to V.
The unit of current is the ampere, named after Ampère, and abbreviated to A.
The unit of resistance is the ohm, named after Ohm, abbreviated in this case with the Greek letter Ω, pronounced omega, (not omeega).