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().