There are 3 types of commonly used blood gas tests:
Arterial Blood Gas (ABG)
The best test, taken from an artery presumably before the blood has been used for anything. Tells you the pH, oxygen level (O2), carbon dioxide leve (CO2), and amount of base. Harder to get than other two tests because only a few spots to get arterial blood (usually arteries are deeper, veins are nearer the surface of the skin) or you need a central line to get blood from.
Capillary Blood Gas (CBG)
Blood taken from a capillary. Just as good as as an ABG except you can't trust the oxygen reading. Capillaries are where blood is used by the body, that is, where oxygen is removed from the red blood cells. So the oxygen level is not reliable in this test since you don't know how much has already been removed by the body. The rest of the readings (pH, CO2, base) can be used with confidence.
So what do you do if you really want to know what the oxygen level is? The pulse oximeter (oxygen level reader on patients' fingers) can be used as a stand in in the majority of cases.
CBG + Pulse Oximeter Reading = ABG
Venous Blood Gas (VBG)
Blood taken from the vein. Effectively the same use as a CBG. Good for all the readings except oxygen level so the rules apply.