They are different.
The ESEE-5 or older RC-5 are identical. The Ontario RAT-5 is thinner, has a choil, and a cheaper coating. It was designed by Jeff Randall of ESEE Knives, but he has no affiliation with Ontario any longer.
The nicer and stronger knife of the two is the ESEE-5 IMO, it is well worth the difference in price.