Take a look at the T-SQL string functions. You can use CharIndex() to check for the presence of a comma and, if present, return a truncated string using Left(), something like
CASE WHEN CharIndex(',', fullOU) > 0 THEN Left(fullOU, CharIndex(',', fullOU)-1) ELSE fullOU END AS lowOU
The simplest way to drop the "OU=" would probably be to wrap the CASE in a Replace().