Double reed instruments, such as the oboe, are played by blowing air over the reeds and causing them to vibrate. Since an oboe has no mouthpiece, using a metal tube to hold the reeds and connect to the body of the oboe, placement of the lips, teeth, and tongue and control of the amount and placement of the air being blown is critical in producing the desired pitches. This is called embouchure - shaping your lips and the muscles in your face to conform to the shape of the oboe's reeds.
Make sure your instrument is working properly. Check that all the keys and pads are working correctly and are completely sealing when their openings should be closed. You may need to adjust a screw or joint between sections of the instrument. Verify that your reeds are positioned correctly in the tube and in your mouth. Beyond those mechanical considerations, remember that humidity and temperature have a significant effect on reeds.
Lots of practice is the ultimate answer to your question. You need to develop your embouchure to the point that you adjust your positioning without thinking about it, automatically changing as you recognize that the pitch you need to play is a very low one.