A number of factors can come into play when designing a stage set for a "Wizard of Oz" production. One is the construction materials you have. The other is the exact script you are using and all the scenes within; not every script is written exactly like the well-known 1939 film, which itself was not exactly like the book. Any production will have multiple scenes in many venues, requiring set pieces you can move quickly.
The biggest factor when creating the Gale farm in Kansas for the opening and final scenes is that everything should be bland. Every piece of scenery should be in faded browns and greys; this will contrast with the bright colours of everything in Oz. The house is the central part of the scene. It should be open-ended with a full view of the inside, as its main focus is Dorothy being trapped inside during the cyclone. The storm cellar, where the rest of the family escapes, can be just a wedge-shaped construct with doors right near the stage wing so these characters can enter and walk offstage without being seen. You can also add a simple pigpen and windmill for more detail.
The land of Oz for much of the first act is divided into different parts: the Munchkin Village, the Scarecrow's cornfield and the forest. These can be represented by small houses, cornstalks and trees on stand-up plywood or foam boards that can be easily painted and lightweight enough for quick transport by stage hands. You will need a post for the Scarecrow to hang on and at least one tree large enough for the Cowardly Lion to pop out from. Placing the Yellow Brick Road on the floor isn't absolutely necessary unless some in the audience can see the stage floor. The exterior of the Emerald City may best be limited to a painted backdrop showing the city in the distance.
Nearly everything should be green in the Emerald City. The most important set pieces include the main doors from the outside and the hallway/doorway to the Wizard's chamber. The image of the Wizard as a giant head may best be done with a projector from behind the audience. You'll also need one small section of the chamber partitioned off with a curtain; this is where the real wizard is hiding.
The witch's castle and surrounding forest should be dark and foreboding; lots of dead trees will set the mood. The castle walls should be highlighted in black and green, with the walls depicting twisted, evil designs. If the budget and materials are sufficient, add one small high chamber atop a staircase, where Dorothy is to be trapped. It will also help to have some sort of trapdoor on the stage for the witch to "sink" into while melting.