If the cells in the veins in a human body need oxygen, how do they get it?
From the lungs. Oxygen is passed into the blood.
There’s a fair amount of oxygen in venous blood. Enough to supply venous tissue. It’s just low in oxygen compared to arterial blood, and it wouldn’t supply oxygen-hungry muscles or nervous tissue.
QtM, MD
If I remember my biology corectly, the air we exhale is around 15% oxygen (compared to 21% for ambient atmospheric air). It’s enough to oxygenate the blood of a victim during rescue breathing, or mouth-to-mouth resuscitation.
Venous oxygen saturation is generally over 50%. Oxygen concentration of exhaled gas generally doesn’t enter into it.
Arterial oxygen saturation should be in excess of 90%.
Ok, we’re leaving out something important here. Yes the veinous blood contain O2 and yes a little of it is absorbed by the inner cells of the inner layer of the vein/artery. But they are too thick to be completely perfused without vessels themselves. Veins and arteries are like ogres and onions (they have layers). So there are these vessels for the vessels called vaso vasorum. The vaso vasorum are like capillaries for your vessels and make sure every cell in the vein get’s its O2
The vaso vasorum does not supply the intima (innermost lining) of the veins. Bear_Nenno is quite correct though, that it does supply the media and adventitia, the parts further away from the venous blood flow, with oxygen. The intima is supplied with oxygen via the venous blood for the most part.
Perhaps a hijack… but, yesterday I was wondering how the cells in my cornea and cristaline get oxygen and nutrients… anyone?
corneal cells get oxygen directly from the outside environment, via diffusion. That’s why you need gas-permeable contact lenses if you wear them for any length of time. Otherwise the corneas get hypoxic and die.
If you had capillaries feeding corneas, they’d affect the vision.
and nutrition is supplied via diffusion thru the inner surface and tears on the outer surface. Corneal cells don’t eat much; they are not real active metabolically.
The cornea gets oxygen from the air, which is why there are rules about when and how long you’re supposed to wear contact lenses.
You mean I breath thru my eyes??? Cool… 
I assume then that wearing swimming googles for too long would be bad too, right?