Apache Camel uses Spring JMS in the camel-jms component, so it support what JMS can do. So if Oracle AMQ has a limitation about 4000 bytes or something, its something you need to handle on the Oracle side.
Basically Camel uses JMS and you need to dive into the Oracle AMQ docs to figure out its JMS interop.
JMS has different message types: Text, Bytes, Map, Object etc. For XML you may need to use either Bytes or Text. You can configure the jmsMessageType on the JMS endpoint in Camel to force a specific type. If not then Camel picks the most appropriate based on the message body type. Eg string => Text etc.
There are other users of Apache Camel that has integrated with Oracle AMQ so it should be possible.