Because of its dependability, simplicity of concealment, and quick reloading, the Colt M1903 Pocket Hammerless handgun was the chosen weapon of OSS operators throughout WWII. 

The Office of Strategic Services (OSS) used a variety of weaponry during World War II, including the Colt M1903 Pocket Hammerless pistol, which was widely preferred by OSS operatives. The M1903 was produced in .32 ACP and later .380 ACP, and was easy to conceal in a coat pocket. It had a capacity of eight rounds of .32 ACP, which was impressive for its time, and offered enough firepower to save lives. The M1903 was also reliable and accurate, making it a great shooter. However, its sights were small and hard to see, and it lacked a slide hold open. Despite these drawbacks, the M1903 remained a popular choice for the OSS due to its ease of use and quick reloading nature. 

Source: https://www.sandboxx.us/blog/the-colt-m1903-the-pistol-of-the-oss/