oxygen sensor question

[89TCGTS]

Is there supposed to be an oxygen sensor on the exhaust manifold? I 'm just wondering cause i just read a thread where someone said that they put a bolt where its supposed to be, so i asked my dad about it and he said that some cars have 2 sensors, one on the intake and one on the exhaust. so is this the case or not.........

[Flashmn]

Yes some cars have two sensors, but they're both in the exhaust pipe, one before the cat, one after.
Some cars dont have it at all, like mine, (although I put it there to get a reading for the o2 meter)