I like to use OpenLDAP to authenticate my Oracle users. It should be possible to fetch at least the password from the directory.
There appearently is something for the Oracle LDAP servers, but would it work with OpenLDAP (plain, from i.e. Debian)?
According to this post, you absolutely can, with significant speed improvements over OID.
Looking at this absolutely ancient post, it was apparently possible back in 2003.
Here's the Archive.Org transcript of the page they reference.
This is a Oracle documentation link about hooking OID up to an "external directory". Good luck.