Can we add OU in SharePoint security group instead of adding users or AD group?
You can't use OU instead of group(s).when we have standard alternative solution:
You can use/create a new group and named it : for example : mega-group1 and grant membership to all other groups there.(in that OU) , then use this new group for permission in SharePoint sites and etc.
I guess a big and complex mistake, force you to use OU instead. and mistake in last month happens for me and this is a link for you link