O-PALM an open source dynamic parallel couple. Since 1996 CERFACS has been developing the PALM parallel coupler, which is currently used for more than 50 research and industrial projects ranging from operational data assimilation to multi-physics modelling, from climate change impact assessment to fluid and structure interactions. It can be defined a dynamic coupler for its ability to deal with situations where the component execution scheduling and the data exchange patterns cannot be entirely defined before execution. Under the name O-PALM, it is now open source (LGPL license). This document provides some highlights on the design of PALM and on the main implementation choices and a brief description of some representative applications