There are a lot of excellent answers how can one simulate object oriented concepts with C. To name a few:
To just give you another example: a fair amount of the x86 Linux kernel is using C as if it were C++, when object-orientation seems natural (eg, in the VFS). The kernel is written in assembly and C (if that wasn't changed in the 3.0 kernel). The kernel coders create macros and structures, sometimes even named similar to C++ terms (eg, for_each_xxx), that allow them to code as-if. As others have pointed out, you'd never choose C if you start a heavily object-oriented program; but when you're adjusting C based code to add object-oriented features, you might.