Bjorn  Hansson  (taragalinas) at BGG wrote this great piece. I wanted to share it with all of you .

This guide is aimed at people with just a little experience of wargames. To give them a glimpse of what OCS is all about. More experienced players will no doubt be able to find exceptions to some of the things I say, but my goal is not to write a complete guide to OCS, more to give an indication of what makes the system stand out. Feel free to add your thoughts on the game.

General
The Operational Combat System (OCS) is, as the name implies, an game system for simulating operational level warfare in the first half of the 20th century. The series rules and most of the games are designed by Dean Essig.

The games are played on classic hexagon maps with dual sided cardboard counters. The time scale is approximately 3½ days per turn. Each hex is about five miles in diameter and the units range from battalion to division – roughly stretching between 300-15000 men. Air units are composed of 20-40 aircraft per counter.

The data on each unit symbolises combat strength, action rating and movement. I.e. a 5-3-4 unit would have 5 in combat strength, 3 in action rating and 4 in movement. Action rating is an indicator of the unit’s training, experience, morale, cohesion etc. and ranges from 0 (very poor unit) to 5 (elite unit). Artillery has an additional number that indicates range of fire. Air units have two values. Air combat and Barrage strength.

OCS places its main emphasis on two aspects of warfare: logistics and mobility.

Logistics – An army marches on its stomach, Napoleon.
In OCS players are faced with one of the most troublesome facts in warfare: your supplies are limited. Major operations require huge amounts of resources – food, water, ammunition, fuel etc. You also need to have the means to deliver the supplies to every unit as the army advances.

There are two types of supply in OCS. Trace supply and Supply points. Trace supply is the type of supply found in most wargames. It is an abstraction of the supply network established by support units to deliver the daily needs to the units.

In OCS trace supply is only used for basic sustenance. The source of trace supplies usually reside off map. It enters the map via a rail line or a port. Units close to a town along the rail line or near the port is considered to be in trace supply. The trace supply network can be enlarged by the use of so called extenders (a network of transport vehicles) or by a headquarter unit. Any amount of units can be supplied within the trace supply network.

When it comes to combat OCS utilizes Supply points. One supply point equals approximately 1500 metric tons of supplies needed to fight the enemy. Every supply point can be divided into smaller units, called tokens. Four tokens equal one supply point. Supply points are physical counters present on the map.

As soon as a unit wants to move its motor vehicles, fire its artillery or engage in any type of combat it consumes supply points. Supply points are also consumed as food if a unit is outside the range of trace supply.

The amount of supply points available to the players each round depend on the historical conditions of the campaign. The games try to adhere to the historical conditions as far as possible.

Supply points usually enter the map the same way as trace supply, but in a more limited fashion. Each physical object (soldiers, artillery, supply points etc) that needs to be transported by rail or sea in OCS requires some sort of transportation. Each player will have a limited number of rail transport points or sea transport points. Again, these try to simulate to historical conditions as closely as possible. On top of that the player will have a number of truck/wagon units that can transport supply points from the port/railroad station to the frontline if the railhead is too far back. Unlike trace supply all of this happens physically on the map.

So, a commander in OCS not only has to think about where he is going to attack. He also needs to think about how much supply he has available and how he is going to get it there. This makes for a much more realistic approach to warfare.

Mobility
OCS really shines when it simulates mobile warfare. In a lot of wargames the frontline contains 99% of all units in the game. This of course is quite contradictory to how real war was fought. OCS forces the players to garrison key locations in rear areas and create a real defence in-depth to protect against enemy breakthroughs. The second line of defence is sometimes more important than the first one. How is this done?

In OCS players have the option of putting some of their units in reserve mode. Being in reserve mode means that the unit gives up most of its mobility in the regular movement phase and instead prepare for movement in a later phase during the turn. The amount of units you are able to put in reserve depends on the quality/doctrines of the army you are commanding and is regulated by the rules – again to simulate historical limitations.

Basically what this system means is that an army can attack to create a hole in the enemy frontline and then have its reserve units storm trough the gap and into the enemy’s rear. This in combination with the supply system, which allows players to transport supply using trucks and wagons deep into enemy territory, forces commanders to defend in-depth and to keep their own reserves ready to be able to react to enemy breakthroughs.

Why I like it?
The reason I like OCS is because I feel it forces me to think in ways that are reminiscent of the line of thought of actual military commanders. It rightfully places the importance of logistics at the forefront of the gaming experience. I cannot attack everywhere at once. I must spend time building up supply dumps for major offensives. If I want to shift focus of my campaign it requires planning. Troops have to be transferred, transport units relocated etc. I also need to consider how I am going to follow up an offensive. Will my engineers be able to convert rail lines fast enough as I storm across the Russian steppes? I cannot just be bold and daring, I have to be rational and cool headed as well.

But I do find the resources I need for an offensive the system lets me do pretty much anything I want to. One of the best illustrations of this is to look at something I haven’t mentioned yet: the war in the sky.

Air units in OCS can do pretty much whatever you want them to. Your active fighters create a patrol zone over your troops; you can perform fighter sweeps to suppress the enemy; you can intercept incoming enemy units; you can hamper enemy movement through “trainbusting”; you can attack ports, ground units, air bases and ships; you can transport units and drop paratroopers or supply behind enemy lines.

The possibilities are, well not endless, but close enough! It allows me to pursue an entirely different strategy than my historical counterparts even though I am placed in the historical context with all its limitations.

The system has its flaws, but IMO it is without a doubt the best take on operational level warfare on the market right now.