Sorry, probably very stupid question but I didn't find an answer to it.
How, by using which flags, can I get the object file (.o), or anything that's not the final executable, like an assembler or C source?
You could check the nimcache directory(the default is on your system's temp path) You can specify --nimcache:path too.
nimcache/ directory contains the .o and .c files. If you could use --asm to generate assemble codes.