I am a big fan of OCS and BCS. Recently a BGG user asked me to compare the two, so I have. After writing my thoughts down I thought it might be worth sharing with others. Gnarled veterans of both systems feel free to correct/point out any errors. I'm not being precious about this. I have tried to focus on the key similarities and differences. I'm not so interested in discussing the whys and wherefores.
Supply
OCS uses both trace supply and on-map supply. Trace supply is a very basic level of supply where, Combat units must be able to trace a path to a supply source or they may lose steps/whole units. This supply source is usually a port or a map edge hex. Rail connections, divisional HQs and 'extenders' are typically used to extend that range.
On-map supply is a very literal form of supply and uses Supply Points (SP). These are counters that are physically moved around the map using mules/trucks and air transport. Supply Points are required if you want to attack, re-fit your aircraft, use barrages or keep units that are cut off from trace supply from losing steps. It is not required for units to defend themselves but their strength is halved if they don't have/choose to use it.
In BCS, there are no supply points to transport around the map. Instead you need to maintain a Main Supply Route (MSR) - basically a line of supply between the map edge, the combat trains (effectively supply hubs - one per formation), and finally to the HQ's. Failure to maintain the MSR leads to step losses and significantly reduces the ability of the formation to activate fully/at all.
Aircraft and Artillery
In OCS aircraft and artillery units are physically represented on the map (although sometimes planes have off map bases too) with counters and specific rules about how, when and where they operate. With aircraft you can do a variety of missions including fighter sweeps, interceptions, bombing, transport troops etc...
In BCS this is far more abstracted. There are no artillery or air units. Instead both players will receive artillery points. 449
Artillery points are either inherent to an HQ’s or can be assigned from High Command. They are used to either kill or suppress units, there is no such thing as ‘disorganised’ status for units (always shortened to DG’d) as there is in OCS and SCS.
Air points can be used by any Allied formation and act in a very similar fashion to artillery points. There is no Luftwaffe because the results of their efforts in this campaign were so negligible. In fact there is no air to air or ground to air combat at all.
Movement
Movement in OCS is far more varied than BCS. A unit can be in a number of modes that determine when and how far a unit moves, e.g. reserve mode, strat mode etc, move mode etc... These are shown by either flipping the counter or placing a marker.
With BCS there aren’t any modes. A unit can be either be flipped to its move side, which allows greater movement or to its deployed side. The deployed side may change how the unit moves, e.g. tracked PZG can switch to leg movement (going on foot) or improve its ability to fight.
Combat
Both OCS and BCS give their units an action rating (AR) which influences how well they fight. For OCS the AR is probably more influential.
In OCS a combat starts with a 'surprise roll' which is modified by the difference between the AR's of the two units and can influence a combat by up to six column shifts in favour of either the attacker or defender.
This makes it less attractive to those that regard wargaming as an accountancy exercise but is great for those that look for a bit of unpredictability. It then uses a series of modifiers to determine the result. As well as the surprise roll, the usual odds (3:1 etc...) and terrain are a key determinates in the outcome of combat.
In BCS the combat is far more predictable and uses a range of modifiers as you would expect based on the AR of unit involved and terrain. Terrain however is nothing like as important for determining the outcome as OCS. There is however a wider variety of attacks a unit can do in BCS and therein lies much of the fun of the game, e.g. engagements (ARM v ARM), regular attacks (assaults), shock attack (ARM v INF) etc...
Strength of a unit only matters in ARM vs ARM. It is not included on Infantry units, which will lead to a bit of head scratching when you look at those counters for the first time. No odds tables are used at all. Radical stuff! 100