1997 bmw 7 series

Hello i have a question…..i recently bought a used 1997 bmw 7 series 162,358 miles from a guy i work with. He told me the only problem is the o2 sensor and he said he drove the car like that for about a year becuase he forgot to change it out. How long can i drive before it starts causing major problems? Can i run my air conditioning? Money is tight wondering if i should get this fixed asap. Thanks