Long before the time in which the game starts, two extremely powerful races, known as the Orions and the Antarans, fought a war that devastated most of the galaxy. The Orions won and, rather than exterminate the Antarans, imprisoned them in a "pocket dimension". The Orions then departed from the galaxy, but left behind a very powerful robotic warship, the Guardian, to protect their homeworld. Whoever beats the Guardian gets military technologies which players cannot research for themselves and the opportunity to colonize the Orions' homeworld, which is usually the best planet in the galaxy in all respects.
Some time after the start of a game, the Antarans, breaking out of the prison dimension to which the Orions banished them, begin sending increasingly powerful fleets against players' colonies, simply to destroy rather than to invade. The only way to stop the Antarans' campaign of terror is to carry the battle to their home universe through a Dimensional Portal.
Despite the game's name, conquering the Orion star system does not automatically win the game. There are three routes to victory: conquer all opponents; be elected as the supreme leader of the galaxy; or make a successful assault against the Antaran homeworld. To be elected, a player needs two-thirds of the total votes, and each empire's votes are based on the population under its control
Star systems have at most five colonizable planets, and a few have none. Players can colonize all types of planets although gas giants and asteroids require the planet construction technology. Colonizable planets vary in several ways, making some more desirable than others:
Population capacity, which on most planets can be improved by terraforming. "Toxic" planets cannot be terraformed.
Ease of growing food, which is important for the reasons described below. At the start of the game most planets are incapable of supporting agriculture, but terraforming can remedy this, except on "toxic" planets.
Very rare "splinter colonies", which automatically join the empire that discovers them and acquire its racial advantages and disadvantages.
Sometimes other features that increase or decrease productivity in one or more of farming, industry, research and cashflow.
The most desirable systems are usually guarded by space monsters, much less powerful than Orion's Guardian but still a severe challenge in the early game, when fleets are small and low-tech
Without food, a colony will starve to death. If an empire as a whole has a food surplus, it can prevent starvation by sending food in freighters, which are produced (in groups of five) like any other ship and require a small amount of upkeep when in use. However, just one hostile warship of any size can blockade an entire system, preventing the delivery of food.
Each player can change each of its colony's output by moving colonists between farming, industry and research, except that natives can only farm. All normal colonists pay a standard tax to the imperial treasury and in emergencies one can set a higher tax rate, but this reduces industrial output. A player can use surplus money to accelerate industrial production at specified colonies, but not to increase agricultural or research output.
Maintaining buildings costs money and so does running an excessively large fleet. Ships of different sizes require different numbers of "command points". These are provided by orbital bases, which are major construction projects for small colonies. This severely limits the size of empires' fleets in the early game, where one can have only one frigate (smallest type of ship) per starbase or one battleship (largest type of ship in the early game) per 4 starbases without having to "buy" command points, which is very expensive.
Research, usually followed by construction of appropriate buildings, can improve all types of productivity including cashflow and command points, and can also reduce or eliminate pollution, which otherwise is a serious constraint on industrial output in the early game.
Falling behind in technology is likely to be fatal. There are eight research areas, each divided into several levels, each of which contains one to four technologies. To research a higher-level technology, one must first have researched the previous level. "Creative" races get all the technologies at a particular level by completing one research project at that level; most races must choose only one technology from each level; "uncreative" races get no choice and the game software randomly selects a technology for them at each level. Players can also acquire technologies by exchange or diplomatic threats, spying, hiring colonial leaders or ship commanders with knowledge of certain tech, planetary conquest, or capturing and dismantling enemy ships, random events and just by stumbling upon it in a derelict craft orbiting a newly discovered planet.
All weapons and some other combat-related components benefit from miniaturization, in which further advances in the technology area that provides them will reduce the size and production cost of these components. Miniaturization also makes available modifications for most weapons, which usually entail a significant increase in their cost and size but can greatly improve their effectiveness in the right situations
Master of Orion II provides a wide range of diplomatic negotiations: gifts of money or technology or even all the colonies in a star system; opportunities to demand such concessions from other players; technology trades; trade, non-aggression and alliance treaties. The diplomacy menu, called "Races" in the game, also enables the player to allocate spies between defensive duties and spying or sabotage against other empires, and to check opponents' technological progress and diplomatic relationships
The designs of colony ships, outpost ships and troop transports are fixed.These three ship types will be destroyed instantly if they travel without an escort and are attacked by anything, even the weakest combat ship.
Colony ships, outpost ships, troop transports and warships benefit from technology advances which increase the travel range and speed of all of an empire's ships free of charge.
Players can design warships, provided they choose the "tactical combat" option in game set-up. One can design a maximum of 5 classes at a time, but can have an indefinite number of classes in operation. Players can also refit ships to take advantage of technology improvements which do not provide free upgrades
Ships can travel to any star system within their range, unlike games such as Space Empires or Master of Orion III where interstellar travel is possible only or mainly via "wormholes" and it is possible to set up easily-defended choke points.
In Master of Orion II, space combat occurs only within star systems, either over a planet one side is attacking or on the outskirts of a system, if one side is driving away the other's blockaders or trying to prevent an enemy buildup. If the defending side has warships and several colonies in a system, they automatically scramble to defend whichever colony is attacked. Limitations on the size of empires' fleets mean that most battles involve only a handful of ships on each side. Ships do not form stacks as in Master of Orion, but move and fire individually.
At the start of a game the gamer chooses whether all space combat should be "tactical" (controlled by the player) or "strategic" (controlled by the software), but choosing strategic combat prevents the gamer from designing his/her own ships. In tactical screen the gamer can use the "Auto"