The obvious answer is to oil it. But until then, is there any way to open it without the terrible creaking sound?
Once in a while it has opened quietly. I think putting pressure on certain parts of the door helps.
You should lubricate the hinge and doorknob mechanism with oil. GT-85 or other teflon-containing lubricants are the best. If you're not in a position to do that:
Take light, slow steps as you are opening the door. That way, no one will hear the squeak of the floorboards and/or tile as you enter the room.
Be careful when turning the doorknob because some doorknobs can be very noisy as you turn and let go. If it is noisy, turn very slowly and let go slowly, only releasing when it is back in its original position.
Turn the doorknob so that the least rotation is needed. If the hinges are on the right, rotate the doorknob clockwise. If the hinges are on the left, rotate the doorknob counterclockwise. This holds true whether the door opens toward you or away from you.
After you turn the doorknob, press the door upward and then toward the hinges in rapid succession as you open the door. This will help to reduce the noise the door makes as it separates from the jamb.
Jamming the door upward and toward the hinges immobilizes it and makes the chance of squeaking unlikely.
Doors are sticky and noisy when opened? For obstructions near the door.
sand down or strip the offending parts until they clear each other without touching (and possibly repaint with a single coat if you need to). I'd expect sanding only to "rough it up" to increase the friction, not make it better.