What is an Embouchure? The term comes from the French word, bouche, meaning mouth. The embouchure is the position of the lips, tongue, and teeth in playing a wind instrument.
The OBOE embouchure is formed by rounding your lips into an Ooooh, or whistle shape, which gives us a flat chin (1- round oooh), not moving our jaw placement from that whistle; then rolling the lips in (2- rolling the lips in) until your corners are tight (3- corner point inward and forward). To play with this kind of embouchure your reed must be stable and up to pitch.
Practice your embouchure in front of a mirror. Start with a whistle or "oooh" shape. This will cause 1- your chin to point to the ground, and 2- your jaw to drop slightly, just enough. From here practice rolling the lips over the teeth only to the point that the jaw does not move. This is very important! Do not let the lips roll in so far that the mouth takes an E shape, with the corner pointing to the ears. This is not correct. There will most likely be some red part of the lower lip outside of the mouth, and this is okay. Strive to pull your upper lip down as much as possible without moving your jaw from your original "oooh" position.
When you can easily roll in your lips from a whistle or "oooh" next work on 3-pointing your corners inward and forward, like a dog's snout. Think of a drawstring bag pulling in tighter and tighter in a round motion- this is what your embouchure should be doing. You should feel the corners get tight, but you should not feel like your upper and lower jaw are closing down.
Chin is pulled down, flat, pointing to the ground. The jaw is open just enough to allow the chin to point down. The corners are tightening inward and forward.