Is it ok to have flexible oil pipes exposed outside of the appliance casing?

No, British Standard 5410: Part 1: 1997 requires oil supply pipes within a building or above ground externally to be constructed of steel or copper. Unless the flexible oil pipes have an equal degree of fire rated resistance to that of steel or copper (to be declared by the manufacturer) it should remain concealed within the appliance casing where it is protected by a remote fire valve.