Ohm symbol in latex

You can use \usepackage{siunitx}. After that just input as \si{\ohm}. If you want to use a specific value it is better to mark it up like \SI{1}{\ohm}.

Just use \Omega since ohms symbol is uppercase Greek letter omega.

Since the ohm symbol is just the non-italic greek omega letter, you can as well use the command \textOmega from the textgreek package, which is in the large symbols table.