I remember as a kid going out to the park with my family, finding a big, impressive tree and spending hours sketching it with my colored pencils. Back then I tended to draw each tree with the same generic bark pattern of interrupted wavy lines with the occasional knot to make things interesting. Now I know better: most trees have a distinctive bark pattern which can mean the difference from a nice, generic tree to a recognizable oak.
Find an oak tree. Drawing from life is the best way to work: you can closely see each nuance of an object and move around until you find the most visually stimulating composition to use as your subject. Trees are especially good models since they seldom move! If going to your backyard or local park isn't an option, the Internet is a wonderful resource for reference pictures.
Look at the bark. I know this probably sounds obvious, but the most common artistic dilemma is when the things we draw just don't look "right." This happens because we try to draw what we know is there instead of what we actually see. Training yourself to see clearly and draw only what you see will make your drawings more successful.
Sketch your basic shapes with a light pencil. What I see when I look at oak bark is a series of parallel rectangles and skinny trapezoids so that will be the shapes I start with. Keep definite columns in mind as you draw but allow enough deviation to avoid looking like a grid of bricks. Look at your reference tree or image to notice that some pieces have little jagged fingers that slip between other chips and some that overlap. Add a few of these details into your basic shapes to make things interesting.
Add some dimension. I left some spaces between most of my initial shapes because each piece of oak bark is pretty thick and fits together like a mismatched jigsaw puzzle. Using a heavier pencil (I prefer charcoal), add the edges of the chips of bark to the right and below each piece. This is also a good time to go back over your original shapes and rough up the edges a little so they look more natural. There will still be gaps between the now three-dimensional bark planks: this is the rest of the tree peeking through. To keep the different layers straight, go ahead and darken those gaps with the charcoal pencil as you move across the drawing.
Separate the sides from the top of each piece of bark. Using either a lighter pencil pressure or hatching, shade the side and bottom of each piece of bark to give them a medium value between the top of the bark and the visible tree background.
Add some texture to the bark. We have dimension on the sides and background at this point but the top of the bark is looking a little boring. Nature is seldom boring, so go back to your reference tree or image and notice that each piece of bark has ridges, cracks and varying surface textures going on. Try to reproduce a little bit of that with lines and shading onto each piece of bark to give the finished product a more realistic look.
Nature loves asymmetry so try to keep your bark balanced but not necessarily even.
Charcoal smudges easily and can transfer onto your skin and back to the paper. Periodically clean up these stray marks with an eraser and then brush the eraser bits away with a soft brush or feather duster to keep from causing more smudges.