O2/boosting problem Options

[90GSX]

Ok my car has been throwing a C/E light for some time, well it comes on for a few days, goes off for a few days, and so on. Well for a while now i have felt a powerloss from when i bought the car it seems. My factory boost guage only is reading 4-5 PSi and never any higher. Well after playing with my pocketlogger, the light says its code 11 O2 sensor. Does that mean its bad or is something else making it read wrong? I logged a run and the front o2 reads 2.5v all the time, never changes. the rear o2 read 0.00v-0.13v peak on a run. I get about 43 counts of knock about 6500-7k RPM, it reads ~0 till i hit 3k-3500 rpm. My car is totally stock, and i checked the piping and dont see any visible leaks and my question is will a bad o2 make you loose boost and is the o2 sensor itself bad? They are 118 bucks for a new one so i wanted to cover all other options before i got a new one. I have 176k miles, and i know at one point the previous owner replaced it already, but i dont remember when, i will have to check the paperwork. any help will be appreciated. Oh and another thought of a boost leak would be it might possibly be leaking into the crankcase, the dipstick pops out sometimes and also the "breather" completely comes out also. My oil pressure reads about a quarter inch from the 2nd line on the guage when i get on it, i have a feeling that probably isnt accurate considering the miliage. Anyone got any ideas? thanks