FWIW, I don't think that the intent of the 1939 film is for Oz to be just
Oz was the part in Technicolor, whereas "real life" was in black and white. Furthermore, the greatest endearments and most frightening perils occurred in Oz. The tornado is scary and impressive, but the Wicked Witch is scarier. The Scarecrow, Tin Man, and Lion are quite familiar, and yet we end up knowing the farmhands almost not at all. In every tangible way within the film, Oz has greater reality than Kansas.
In the end, Dorothy can't believe that Oz was just a dream. Why are we supposed to put our trust in the words of characters rendered to us in only black and white, whom, again, we hardly know at all? The film is dedicated to the "Young at Heart". We can easily take that as a declaration that we aren't supposed to believe Aunt Em when she says that Dorothy "just had a bad dream", since what Em is saying is the "grown-up thing" to say.
Granted, there may not be much room in the film for Dorothy to have physically
gone to Oz. However, the way the image splits in two after Dorothy gets hit on the head (around the 2:00 mark of this video
), in my "head canon", I've always interpreted that as physical reality going one way and Dorothy's true inner reality going another.
I refer you to Tolkien's essay, "On Fairy Stories."
That's now on my to-read list.