You could start with standard Oracle auditing to catch that access, but a knowledgeable admin may well then doctor the audit tables, or disable auditing.
So you could then consider something like Database Vault
http://www.oracle.com/technetwork/database/options/database-vault/index-085211.html https://docs.oracle.com/cd/B28359_01/server.111/b31222/dvintro.htm But that's a fairly big hammer to be wielding. In particular, ultimately there is going to have to be *some* sort of trust relationship here. Because if they are admin to the server, no matter what controls you put in place in terms of access to the *data*, nothing can really stop a malicious admin from (say) wiping a hard drive and ker-splat.
Similarly, they could simply take the files (because they have admin rights) to another machine. So now you'd need encryption as well, ie
http://www.oracle.com/technetwork/database/options/advanced-security/index-099011.html or you could Data Guard the database back to your site so you could monitor a copy, etc.
Very quickly, you're getting into a fair bit of complexity here. Better to have the basic trust agreements in place in my opinion.
Others welcome to share their views.