OOB = Order of Battle. In broad terms they refer to a country's military setup and hardware. In game terms, they refer to the country files (.obat files, in the OOB folder in the game directory) which hold this information.
In game terms, OOB = Order Of Battle = files that store all the information about nation's units, weapons and formations. Easy to edit, much harder to edid without upsetting someone or something
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This post, as well as being an ambassador of death for the enemies of humanity, has a main message of peace and friendship.