The doors are at least somewhat sentient, and are capable of learning, recognizing faces, and taking commands. During Harry Potter's third year at Hogwarts, Professor Flitwick used a picture to familiarize the doors with Sirius Black's appearance so that they could refuse to let him inside if he showed up (PA14).
The doors face out toward the west (PA21).
Oak is a symbolic choice for the material of the Hogwarts entrance. Hagrid's wand was oak (PS5), and it is Hagrid who brings Harry into the wizarding world and his happier new life at Hogwarts. Hagrid and the news of Hogwarts were surprises that changed Harry's world for the better, and according to Unfogging the Future, acorns (the fruit of oak trees) represent unexpected good fortune (PA6). Oak symbolizes protection (JKR), and Hogwarts certainly serves Harry and others as a place of refuge in a variety of ways. -BB
The door to the headmaster/mistress's office is also made of oak (GF30). -BB