Rule: Or so is only used in approximations of quantity or measurement.
Examples of errors:
(1) If you really want to go to the theater or to a concert or so, you always can.
(2) He taught in a Mittelschule or Gesamtschule or so.
English or so is used in sentences like
I've got thirty dollars or so.
He'll finish in a year or so.
whereas German oder so can refer to approximations of quality or kind as well as of amounts or quantity, as in (2) or
Gehen wir ins Theater oder so?
The German expression is thus equivalent to English or something/somewhere like that, or some such thing, etc.