I think that maybe the author felt like finding the end of a loop stood out much better that way, instead of trying to distinguish the ends of conditional blocks from the ends of loops. If you're used to poorly formatted code in an ML PhD program, it does make some sense. I've worked with a bit of academic Fortran code for my mechanical engineering degree and can sympathize. Also, D and O are shorter reaches on the keyboard than { and }.

Yes, "OD" was definitely just "}". DO might have been ") {" to allow

  while (a < b DO
     ...
  OD