OPL (“Object Per Line”) - format was created to allow easy access to and manipulation of OpenStreetMap data with typical UNIX command line tools such as
awk, or typical scripting languages such as Python, Ruby or Perl. It is also great for writing compact test cases.
In an OPL file each OSM object is on its own line with a newline character at the end. Each line contains fields separated by spaces.
This makes some ad-hoc OSM data manipulation easy to do, but it is not as fast as some specialized tool.
OPL - format description on osmcode.org