Authenticating users in OpenLDAP from multiple OUs

I am new to openLDAP and trying to figure out the best way to layout my DIT. The problem I'm running into is many of my applications want to use a specific baseDN in which to look in for authenticating users, however many of my users are spread out in different Organizational Units (ou=managers,dc=ex,dc=com ou=accounting,dc=ex,dc=com etc).

An example is an in house web application. The login page wants to build a bind DN based on the userid and a given OU. I don't see a way to specify multiple OUs so the only way I can get everyone to login is specify the base DN as being the root of my DIT (dc=ex,dc=com). Authentication takes a good 5 to 10 seconds to look through the entire DIT.

Is there a way to create a symlink or something in one OU which points to an actual user account in another OU? This way, I could create an OU called "ou=webapp1,dc=ex,dc=com" and just refer to users from the ou=managers and ou=accounting OUs without actually duplicating user info.

Take a look at this answer but ultimately it seems like there is no way to make users part of multiple ou's