Lebanese army uses former U.S. OPFOR M113 APCs.

Pictures have just popped up on Caesar@Ninja998998’s Twitter account, showing two of the M113 APCs that the U.S Army had modified to make them somwhat look like BMP-1s and to use them as OPFOR (OPPosition FORce) vehicles for training in the 1980s and 1990s. They were sold to Lebanon which apparently (these pictures are not dated) continues t use them.
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M113 APC modified by the U.S. Army for OPFOR training purpose in the 1980' and 1990s, then sld to Lebanon (Picture source: Caesar@Ninja998998’s Twitter account)

An opposing force (alternatively enemy force, abbreviated OPFOR) is a military unit tasked with representing an enemy, usually for training purposes in war game scenarios. The related concept of aggressor squadron is used by some air forces. The United States maintains the Fort Irwin National Training Center with the 11th Armored Cavalry Regiment serving in the OPFOR role. Fort Polk's Joint Readiness Training Center (JRTC) is another major training site typically reserved for light infantry units, and the OPFOR are the 1st of the 509th Airborne Infantry Battalion.

The Army's Joint Maneuver Readiness Center (JMRC, at Hohenfels, Germany) has the 1st of the 4th Infantry Battalion as their OPFOR. Other major units include the First United States Army which consists of 16 training brigades that often also serve as OPFOR. At a basic level, a unit might serve as an opposing force for a single scenario, differing from its 'opponents' only in the objectives it is given. However, major armies commonly maintain specialized groups trained to accurately replicate real-life enemies, to provide a more realistic experience for their training opponents. To avoid the diplomatic ramifications of naming a real nation as a likely enemy, training scenarios often use fictionalized versions with different names but similar military characteristics to the expected real-world foes. During the Cold War, opposing force units employed Soviet military doctrine and simulated Soviet equipment; since the fall of the USSR, US opposing force units have become more flexible to represent a wider range of opponents.

M113 APC modified by the U.S. Army for OPFOR training purpose in the 1980' and 1990s, then sld to Lebanon (Picture source: Caesar@Ninja998998’s Twitter account)