diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -156,9 +156,10 @@ def andset(repo, subset, x, y):
     return getset(repo, getset(repo, subset, x), y)
 
 def orset(repo, subset, x, y):
-    s = set(getset(repo, subset, x))
-    s |= set(getset(repo, [r for r in subset if r not in s], y))
-    return [r for r in subset if r in s]
+    xl = getset(repo, subset, x)
+    s = set(xl)
+    yl = getset(repo, [r for r in subset if r not in s], y)
+    return xl + yl
 
 def notset(repo, subset, x):
     s = set(getset(repo, subset, x))
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -369,3 +369,8 @@ issue2654: report a parse error if the r
   hg: parse error at 2: invalid token
   [255]
 
+or operator should preserve ordering:
+  $ log 'reverse(2::4) or tip'
+  4
+  2
+  9