In 12c, changing the logging settings for Service Bus is done via the EM console. However, this only works for the Admin server. If you're using a clustered domain and need to enable debug on one or more managed servers, you must update the log configuration by hand. This is true for Service Bus whether running On-Prem or as part of a SOACS instance.
The logging settings for a given managed server are located in the following file <domain_home>/config/fmwconfig/servers/<server_name>/logging.xml. To enable debugging for a specific area, you must add the appropriate logger to the file with a level of "TRACE:32". For instance, to enable transport debug, add the following element as a child of the <loggers> element, at the bottom of the logging.xml file:
<logger name='oracle.osb.debug.transports' level='TRACE:32'/>
Log output is written to the managed server's diagnostic log file under the <domain_home>/servers/<server_name>/logs directory. For instance, a managed server named osb_server1 would write to a file named osb_server1-diagnostic.log in the logs directory.
The logging configuration is re-read periodically, so there is no need to restart the server for the changes to take effect, which is a benefit for production environments. However, be aware that debug logging will have a performance impact due to the amount and type of data being logged.
For a list of available debug loggers in Service Bus, consult the following documentation: