Oxygen is so bare metal on the inside that it should not need to rely on external tools for that.
It's a very long time when I had to handle this but IIRC the ico and bmp resources inside an exe are nothing else that headerless raw ico and bmp files.
This said, reading-in an ico/bmp file, stripping the header and placing the remaining binary data into the resource section should suffice.
The only thing to keep in mind is that not only the ICON resource but the ICON_GROUP needs to be populated... I think
(not sure if the ICON_GROUP just needs the entry but no binary data though - have to search for very old code and look into it...)