Missions in OFP's original version take place in one of three vast islands (14x14 kilometers each). The islands contain towns, roads, signs, airports, forests, shrubs, rocks, mountains, beaches, open plains and more. Missions can take place in a variety of weather conditions and at any time of the day or night. There are no official OFP winter, snow, desert or jungle maps. However, everyone's having a blast with unofficial player-made winter, desert and jungle island addons.
There is another fourth island, the training island, which is bare of grass and foliage. This is generally not used in official OFP missions. However, some user designed missions do make use of the training island and there exist some customized player-made versions of the training island.
OFP: Resistance introduces a fifth island, with new and more detailed textures and a larger land mass than any of the original OFP islands.
Speaking of day and night, as the time changes while playing a mission, so do the positions of the sun, moon and stars. It's all quite accurate, actually, with the islands being based at a latitude of 40 degrees north and standard time in effect. You can watch sunrise or sunset and the tide going in and out accurately. You can even navigate by the constellations at night. Impressive, ain't it?