Trying to understand OAL

What does the OAL of a bullet actually do? Does it aid in the feeding of the bullet and or regulate the case pressure?

You mean OAL? (Overall length)

It can affect several things. If the bullet is seated too long or short then it can indeed have feeding issues. If seated out too long, it can also prematurely contact the rifling which will cause a spike in pressure.

The most important thing though is that the shorter the OAL, the less case volume is available. Less case volume = more pressure (think about it - same amount of gas generated, but held in a smaller space).

If you're new to reloading (which by your question I take it you are), then I'd suggest just sticking to whatever OAL is listed in the manual. Once certain things start falling into place and you get a better understanding of the dynamics, then you can start to adjust OAL.

Mr Morden +1
You didn't mention for rifle or pistol.
Especially in pistol with straight wall cases (like 380, 9mm, 40 cal, 45 acp), the bullet nose shape can directly affect what oal a pistol can feed into the chamber and seat on the case mouth. Just because there is load data at a given oal, doesn't mean your pistol can use it. Pistols have to be checked to see what oals they will accept, and then find load data that will fit that pistols chamber. Straight wall cases "SEAT" on the case mouth. If they hit the rifling before they seat, they either wont feed or will develop dangerously high pressures when fired. Hope this makes sense
"Mr Wobbly" puts out excellent diagrams to help explain the affects of oal. The middle RN bullet will seat and fire safely.
https://lh6.googleusercontent.com/-u4qAeVHifGI/TUQpFgf_jeI/AAAAAAAACu0/6qffBqY_gOU/s720/Bullet%2520Shapes.jpg

The link below provides a detailed explanation of OAL.
http://www.larrywillis.com/OAL.html

I'm curious, have you ever reloaded before, do you own any equipment, have you read a reloading manual?

OAL of a bullet?

I think he means of a cartridge...COAL.

Thanks for all the info so far. I did mean to type OAL and the title is a typo and don't know how to fix it. I have reloaded in the past and i'm reloading now. Anybody can read a set of instructions and pull a handle. I'm just wanting to understand what I am doing.

For an auto pistol cartridge, OAL can make a difference in feeding. It can also sometimes make a noticeable difference in accuracy as well.

For revolver calibers you seat to crimp into the cannelure or crimp groove, and that is the right OAL for that bullet in that caliber.

How deep a bullet is seated can affect the pressure a little, such as a .020 difference in OAL in .45 Colt (A low pressure caliber with lots of case volume), vs a lot for a bullet seated .20 deeper in 9MM (A high pressure caliber with low case volume).