I will give your question my best shot until one of the other official guys at ODGW is able to provide an answer.
I can't talk about how the stats for the various tanks and such were arrived at specifically. There are a few I have some disagreement with but overall I think they are fine. Feel free to change certain stats if you feel they are out of line in some way.
OM-1 represents the vehicle remaining stationary for an entire turn to engage enemy vehicles in fire combat. Some factors that determine the likelihood of hitting the target in the game are optics, turret layout, stability of the firing platform and muzzle velocity of the weapon among others.
This last is important in that the faster muzzle velocity means that the round will head directly for the target rather than being lobbed at the target which is the case in some of the early war tanks with the shorter gun barrels and lower velocity.
As the war progressed, the muzzle velocity of main tank guns increased. You can see the difference in the hit probability of a Panzer IV F1 with the short barrel against say a Panzer IVH with the longer, higher velocity gun. The higher velocity generally means that you don't have to calculate as much superelevation into the gun to hit what you are aiming at. This is before the advent of ballistic computers and superelevation mechanisms that came along for modern tanks.
Some tanks in the game do have negative OM-1. This reflects what I have indicated above. Of course, as you get closer to the target, your hit probability increases. Conversely, as you move further away it decreases.
Generally speaking, shorter guns are less accurate than longer guns. Think of a pistol vs. a rifle. The rifle is accurate out to longer ranges.
Crew quality is also a big factor in the game. A veteran or elite crew has a better chance to hit than a green crew and this is reflected in the stats. Also don't forget the "extra" veteran shot for those guys.
All these things combine together and I have been generally happy with most of the vehicle stats as they are with a few minor exceptions.
Have a great day!