A contact lens covers the cornea of your eye. The cornea has no blood supply and gets all its oxygen from the outside air. Any contact lens is a barrier and reduces the oxygen available to your cornea. Virtually all the contact lenses developed in the last few years (like Acuvue Oasys, Air optic Night&Day, or Biofinity) contain silicon which dramatically increases the oxygen to the cornea. For example, the new Acuvue Oasys transmits oxygen five times faster than the old Acuvue II.
If a cornea doesn't get enough oxygen for a prolonged period it can slowly develop neovascularization. This is a painless slow growth of new blood vessels into the cornea. This is undesirable and permanent. Neovascularization cannot be allowed to develop far enough into the cornea or it would cause permanent vision problems. Fortunately this happens slowly enough that it can be diagnosed in the yearly eye exams necessary for all contact lens wearers. The other main risk of oxygen shortage is an increased risk of corneal ulcers. People who sleep in their contacts lenses are at an increased risk of this particular problem. A corneal ulcer is a very painful condition where the eye gets red and very light sensitive. Corneal ulcers must be treated rapidly to prevent permanent vision loss. Increasing the oxygen to the eye helps decrease the chance of both these problems.