OC4J 11 supports EJB3. So, you can use EJB3 beans with Seam if you deploy there. See the examples/jee5 example for more details.
If you are stuck with J2EE 1.4 version of OC4J, you cannot use EJB3 session beans. Instead, you use POJOs with Seam managed transactions. They are really the same as EJB3 session beans except that they do not require EJB3 annotations (e.g., @Stateful and @Remove) and do not require interfaces. So, they are simpler than EJB3 session beans. See the examples/jpa example to see how Seam POJO + JPA is used in J2EE application servers.
For this purpose OC4J 11 counts as a Java EE 5 application server.
When I say "J2EE", the emphasis is on the "2", not the "EE".
I know that some people still use the outdated "J2EE" label for describing all EE appservers, but this is incorrect usage.