Oil preasure will normaly be lower at low RPM's. If it is getting below a safe level you first should check it with a accurate oil preasure gauge. The factory oil preasure gauges are often known for not being very accurate.
If the preasure is low there are multiple thing's that can cause it.
1) How many miles are on the engine and how well maintained is it? If it is a high mileage engine or poorly maintained it could be oil preasure loss due to increased wear on crank and cam bearing's. As the bearing's wear the gap between the bearing and crank or cam increases and consumes a ever increasing amount of oil preasure. Think of it like this. Take a full soda can and shake it up. Take a small pin and poke a small hole in it. It is alot easier to keep the preasure in the can the same with only a small amount of shaking. If you punched a large hole in it, you wouldn't stand a chance of keeping the preasure up. Basically the same thing happens with engine bearing's.
2) Have you changed oil viscosity to a lower viscosity oil lately? Lower viscosity (ie. going from a 10W/40 to a 5W/20) will lower operating preasure and be even more evident at idle. It will also become worse at operating temperature as the oil thin's out the hotter it get's.
3) It is possible it is the oil pump but that does not happen often in newer vehicles.
Don't take chances with low oil preasure. If you can't diagnosis it yourself take it to a professional mechanic and have it checked out.