Can we add OU in SharePoint security group instead of adding users or AD group?
No, we can't.
In general, in AD, OUs are used for structuring the AD and apply group policies. Security groups are used to set permissions on files, folders etc. That's the logic why they haven't made possible to add OUs in SharePoint groups.
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