Is this an old Windows version? XP? Then I think you need an update for PCL support.
http://support.microsoft.com/kb/2600211
If you want to run the library without patching you must build OxyPlot yourself - you can change the build target framework for the OxyPlot .NET 3.5 solution.
http://support.microsoft.com/kb/2600211
If you want to run the library without patching you must build OxyPlot yourself - you can change the build target framework for the OxyPlot .NET 3.5 solution.