A lot of people both internal to Oracle and customers keep asking about when should OWSM be used vs. OEG. Sometime back I posted Oracle's vision for layered SOA security

Here is a quick summary:

Use OWSM in Green Zone

Use OEG in Red Zone (DMZ)

If you need end-to-end security in which case they will want both OWSM and OEG. This is the topology I would recommend for most customers.

If you need only Green Zone security - then use OWSM in conjunction with Oracle FMW products like SOA Suite, OSB, ADF, WLS, BI, etc both on the Client Side and Service Side (assuming you are using FMW technologies for both Clients and Services).

If you need only Red Zone security - then use OEG on the Service Side. You can use OWSM for the Client Side if you are using FMW to build your clients.