OSG is short for “oil sight glasses” or “oil sight glass”.  Like most abbreviations, it is unknown when or where this began.  An oil sight glass will be an early indicator of wear on the bearings in a pump.  As a result, the pump can work more effectively.  Use an OSG  on a pump or turbine or gear box.  As a result, an OSG will reveal changes in machinery lubricant quality.  The condition of the oil, if clear or cloudy is an early indicator of wear on the bearings.   The oil sight glass alone is not preventing moisture from entering your reservoir or removing any water that has become emulsified in the oil.