I note that experienced professionals find it hard to break into the Oil and Gas field due to lack of relevant experience. Is it true for everyone?

I got a job at an oil refinery with a nuclear power plant operations background. Any process experience will tranfer over, such as working at a chemical plant, desalination plant etc. With that experience it will tranfer over for a better chance. Starting refining engineering jobs don't require a knowlege of the whole refinery. Design is design. You will start out with small projects like designing simple systems, say a pipe and control valve using autocad. You will work up to senior engineer to start designing systems which require process knowlege, which you will attain on the job. As I stated start at an easier to get into process environment then transfer over.

No it is not true for everyone