My 1996 Altima is slowing down

I bought this car in 2005. When I pressed on the accelarator the car would go. But when the car is in drive it doesnt push forward like it did. Also, when I press on the accelerator now, it takes a while for the car to go like it did before. I dont know what to attribute this to. When I first got the car, I took it to Nissan dealer for a full diagnostics test. And they told me I needed an O2(oxygen) sensor. I don't know if this has anything to do with it. Anyone has any ideas?

Does the motor rev up when you step on the gas? If so then the trans could be slipping. check the fluid. If the motor doesn't speed up then check for a clogged fuel filter or exaust system.

When I press the gas I hear the engine/motor ( I don't know what the difference between the two are really). But it doesnt go. When I first got that car it would go. But now when Im in a stopped position, it takes a while for it to go. I asked this in yahoo forum and this is what I got as a response:
THe egr is exhaust gas re-circulation valve. It puts exhaust gas in the intake manifold's fresh air supply to bring down the burning temperature in each cylinder to reduce toxic emissions. Too high a burning temperature will produce toxic emissions.. Since the valve takes exhaust gas with carbon in it,,,after awhile the carbon fouls up the valve and it starts to get stickey and work slow or not at all. It can really screw up the engine idle.
The mass air pressure sensor is located in the throttle body. It measure the air pressure going into the throttle body. It too gets dirty and doesnt want to measure correctly. The mass air temperature sensor measure the amount of actual oxygen in the air based on the air's temperature. It is located on the air breather before the air filter. Its really a delicate wire theat sits in the air stream and works like a thermometer. When IT gets dirty the dirt acts like an insulator and gives the wrong temperature.
All these things can be cleaned. you dont have to replace them.And they will work fine.
Al these sensors determine the fuel mix going into the engine and therefore how your car runs.
If you went to a mechanic he would replace them because time is money,,and they always replace with new,,, they never clean an old part..

yeah when i press my gas pedal i also hear my engine....Thank god, LOL..(u have an automatic transmission i assume).. N e way its got to be your transmission, it may be slipping...When a automatic transmission starts to go bad, it will start to slip causing the car to "not go" or accelerate..u will notice it didn't "go" like it used to...I hope this could be of some help...