OSUT is a US Army acronym meaning "One Station Unit Training." Normal Initial Entry Training (IET) consists of Basic Combat Training (BCT), in which potential soldiers learn basic infantry and general soldiering skills--rifle marksmanship with the M16A2, grenades, physical fitness, battlefield movements, cover and concealment, etc., but most importantly: adaptation to military life--and Advanced Individual Training (AIT -- yes, the Army loves its abbreviations and acronyms), in which the newly-molded soldier learns the skills of his Military Occupational Specialty (MOS).
However, the Army, in its great wisdom, has for some MOS's decided that this standard ought to be deviated from. Enter OSUT. OSUT trainees do not have a break between BCT and AIT, and the training is conducted in the same place and under the same instructors and drill sergeants. This allows for a unification in the training of all necessary skills. Military occupations that use OSUT include the Military Police (MP's), the Chemical Corps, and Infantry (although some infantry soldiers progress through a normal Basic Training and then join an OSUT unit).