For me current behaviour of or_where is quite misleading and problematic. To show what I mean:

from foo in "foos",
  where: foo.a == "a",
  or_where. foo.b == "b",
  where: foo.c == "c",
  select: {foo.a, foo.b, foo.c}

Will end up as:

SELECT
  foo.a,
  foo.b,
  foo.c
FROM foos foo
WHERE
      ((foo.a = "a") OR (foo.b = "b"))
  AND  (foo.c = "c")

Instead I would expect it to be:

SELECT
  foo.a,
  foo.b,
  foo.c
FROM foos foo
WHERE
      (foo.a = "a")
  OR  (foo.b = "b")
  AND (foo.c = "c")

I can see why or_where was implemented how it was implemented, however I think that the second behaviour would be cleaner.