$strStringEnde=instr(UCase($LDAP),",")

does not do what you want.
at least I believe so.
would guess you are after something like:
Code:

$strTest="LDAP://CN=DrShark,OU=1,OU=2,OU=3,OU=Test,DC=test,DC=dom"
$TestOU = getFirstOU($strTest)
MessageBox($TestOU,"")

Function getFirstOU($LDAP)

	If instr(UCase($LDAP),"OU=")>0
	 	$LDAP = substr($LDAP, instr(UCase($LDAP),"OU=")+3)
	Else
	 	$LDAP = substr($LDAP, instr(UCase($LDAP),"DC=")+3)
	EndIf
	getFirstOU= substr($LDAP,1, instr(ucase($LDAP),",") - 1)
EndFunction
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