I was talking with a co-worker about C and C++ and he claimed that C is object-oriented, but I claimed that it was not. I know that you can do object-oriented-like things in C,
C isn't object oriented. That was the entire purpose behind the ++
As far as a definition of what it takes to be object oriented: check wikipedia.
Personally, if it supports inheritance, encapsulation, and polymorphism then your good to go. Another key here is having nice keywords like class and object tend to help...
Examples of real object oriented languages (not conclusive) are: Smalltalk, Java, c#, Python, Ruby, C++..
Also, it's possible to have extensions to provide OO features like PHP, Perl, VB (not .Net), ...