If you've started up your Oracle SOA Suite 12c managed servers, and even after the managed server is reporting RUNNING and OK, this doesn't mean that the SOA platform is completely up yet.

WebLogic reporting the state of the managed servers

Even after the managed servers are up, each and every SOA composite must be loaded, and you'll find a deploying composite model entry in the log for each composite (which is not lazy loaded) (learn about lazy loading here!).

Only after you've seen the following message in soa_server1.out is your SOA platform truly up:

SOA Platform is running and accepting requests.

And a screenshot:

Confirmation that the SOA Platform is ready and accepting requests