I got to compile part of OpenCASCADE 6.3.0 using MinGW (no MSYS). With part of it I mean only up to Visualization because I did not care about the rest.

I guess compiling 6.5.0 would not be that different. There are a few files that need some small patching, though.

I did it by converting the MSVC project files with Code::Blocks while using MinGW as compiler. Note, however, that OpenCASCADE uses per file settings (include directories and defines being the important pieces). You would need to add those settings to the Code::Blocks project (you can add them at the project level).

Once you have compiled all files, you will end up with another issue: you will not be able to link all of them. Some DLLs will be produced, some others will fail at this step. The reason is that Windows has a very short limitation in how long the parameters given to a command can be. Code::Blocks will try to call the linker with a huge number of file names, and quite long each one, hitting that limit.

It is possible to work around this issue by creating a special file that MinGW's linker can use instead where all the object files to be linked are specified. Another work around is to rename all object files so they have a short name, put them in the same folder, and hope you can manually link them. In both cases you will have to get dirty with the command line.

That fits the requirement of no MS VS installed, and even no MSYS.