If you have OFP: Resistance, you'll be able to externally set a mission's view distance before playing the mission. For all earlier OFP versions, a mission's script must be manually modified to define or update the mission's view distance setting.
OFP: Resistance introduces a new VIDEO OPTIONS menu setting, VISIBILITY. This determines the maximum viewing distance you can see during any given mission.
The default value is 900 meters. A lower value improves OFP's overall graphics performance because less terrain detail has to be rendered on the screen. A higher value will render terrain at greater distances at a cost of utilizing more CPU and graphics intensive resources. Obviously, choose a setting that's appropriate for your computer.
OFP missions can specify this internally through the "SetViewDistance" command. In versions of OFP prior to OFP: Resistance, the default value is 900 meters. In Resistance, a mission's SetViewDistance value will override the value set by the VISIBILITY setting in the VIDEO OPTIONS menu.
Keep in mind that while increasing visibility settings improves your ability to site the enemy, it's benefiting the AI enemy soldiers against you just the same. Then it's just a question of who plays better - you or OFP's AI soldiers?