Half-Life: Opposing Force/en
Half-Life: Opposing Force is the first official expansion pack for 
Half-Life. It puts the player into the role of Cpl. Adrian Shephard, one of the military specialists of the Hazardous Environment Combat Unit sent in by the government to contain and silence the Black Mesa Research Facility. Shephard travels through new and familiar areas of the Black Mesa facility, encountering new weapons and NPCs.
Like Half-Life, Opposing Force's name holds a scientific value. Although it means literally the "opposing force" to Freeman and Xen, the name also links to Isaac Newton's Third Law of Motion. The game itself is often abbreviated to OpFor and Op4.
Contents
History
Opposing Force was developed by the external and acclaimed company Gearbox Software. Gearbox aimed to further the story of the Black Mesa incident and show exactly how the situation was ultimately resolved. Opposing Force was originally released in 1999, and had since been named 'Game of the Year' by the Academy of Interactive Arts and Sciences.
Content
As Opposing Force returns the player to Black Mesa as a soldier, Opposing Force's content differs from Half-Life's to maximize the role of the HECU, giving it a stronger presence and a more realistic feel. As such, Opposing Force introduces different types of soldiers: in addition to the shotgun and automatic wielding troops of the original game, Opposing Force includes soldiers armed with light machine guns as well as engineers capable of cutting through sealed doors and medics who can heal both the player and friendly NPC's.
Gearbox also expanded the number of participants in the incident. Now included with the Black Mesa personnel, HECU forces and Xen invaders are two new groups. One of these is the mysterious Black Operations, who are specialist human forces that are faster and more agile than HECU troops. Only encountered twice in Half-Life - female assassins attempting to kill Gordon Freeman - the Black Operations are now in the facility in force, targeting anything that is still alive, including the player and fellow HECU members. The female assassins from the original game return, working in conjunction with new male soldiers and snipers.
The other one of these groups is a completely new hostile force known as Race-X. These creatures are not part of the Xen forces, and will in fact attack Vortigaunt troops as well as human ones. It has been speculated that Race-X is actually a first strike recon force of the Combine. This theory was fueled primarily by similar texturing between the Race-X Shock Trooper and the Combine Advisor in Half-Life 2.
A number of new weapons and models have been introduced to further better the game. These include the M-249 SAW light machine gun and M-40A1 sniper rifle referred to above, as well as a Desert Eagle (which is used in place of the .357 Magnum in single player, but the two separate weapons are available in multiplayer) and a host of alien and experimental Black Mesa weapons. New NPC models include redesigned HECU soldiers to accommodate the new features, Black Operations soldiers, the new Race-X species and a number of new NPC's made specially for the Boot Camp, Opposing Force's version of Half-Life's Hazard Course.
Gearbox composed their own soundtrack for Opposing Force, which was later used again in 
Half-Life: Blue Shift. Yet for unknown reasons, this soundtrack did not migrate with the rest of the game to Steam.
Chapters
Half-Life: Opposing Force has 13 chapters (14 with Boot Camp). Players can also travel to Xen (displays as C4A1TITLE) in We Are Not Alone, but this will quickly result in player falling into a void.
