The pulse is a result of the heart pumping and this is essential for life. The pulse can be fast, slow, weak or strong for example and is a result of the strength and speed with which the heart contracts. A low oxygen level in the blood usually means that the lungs are not working normally and are unable to absorb enough oxygen from the atmosphere. This can be from many causes like emphysema, scarring, pneumonia, asthma and other rarer causes. In congestive heart failure where the heart does not pump effectively fluid can back up into the lungs and cause low oxygen. In general a person with a pulse and a low oxygen level has a problem with the respiratory gas exchange mechanism but this can be from other organ system problems also. As you have not given the details of the situation I can only provide a general response and you should approach your doctor for a more specific response as it applies to your situation.