(Under construction.)
OPL is Psion's own programming language. An editor and runtime enviroment for OPL is built into most EPOC R3 and R5 platforms. It continues to live on after Psion's pull out from the PDA market as the opl-dev open source project and can now be used on Nokia 9200 and Symbian OS 60 devices.
OPL can be extended using OPX modules (written in C++) some of which are made available by third parties. For example Andy Clarkson's (Twiddlebit) Gprinter.opx plays an essential role in enabling Demarco5 to print its vector graphics.
Custom OPXs can also be developed using the C++ development tools discussed on my page here.