When is C a better choice than C++? [closed]

Question

I can't imagine that there's any reason not to use the OOP features of C++. It's is just as fast as C, and - what's more important to me - it's transparent, just as C. (I mean "transparent" that I know, what is my source is compiled to. Or, I think, I know.)

Is there any reason not to use the power of OOP in C programming?

I accept only embedded systems or other restricted platforms as exceptions, where no such memory management is available, which is required by C++. I also accept operating system development. Or programs started writing before C++. Okay, that's all.

UPDATE: the question is just about C vs C++, not C/C++ vs other languages. I'm using C/C++ just in "final case".