I hear this now and then, and it has never made sense to me.
Obviously the objects CAN be written out for C to digest, that's how C++ was first implemented (and there are still C++ compilers available that do this).
The word easily is the part that I just can't swallow. I see examples of how some of the tenets of OOD and OOP may be implemented in C, but consider these two...
There is without a doubt no possible way to implement RAII in C, unless the concept is trivialized down to an integer on the stack or something like that.
There is no way to implement an equivalent to smart pointers that provides any leverage remotely analogous Java's references, which most C++ smart pointers do.
These two alone are capable of magnifying the solidity of code.
Now, I actually agree one can generate objects of a rudimentary kind, but as the complexity of design increases, it becomes a burden, not a leverage, to continue with it in C.
I've yet to find anyone provide convincing examples that demonstrate the contrary.
To that extent, there is a way to code objects in assembler - the way the compiler does it when it processes C++ code, for example