Is this language an OO language? Is it often used as OO language?
Lua is a powerful, fast, lightweight, embeddable scripting language.
Lua combines simple procedural syntax with powerful data description constructs based on associative arrays and extensible semantics. Lua is dynamically typed, runs by interpreting bytecode for a register-based virtual machine, and has automatic memory management with incremental garbage collection, making it ideal for configuration, scripting, and rapid prototyping.
It does support object oriented programming with some difficulty. This chapter in the official guide explains http://www.lua.org/pil/16.html
Lua is fully capable of prototype-based object-oriented programming similar to JavaScript.
Prototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse (known as inheritance in class-based languages) is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as class-less, prototype-oriented or instance-based programming. Delegation is the language feature that supports prototype-based programming.
For more information, see Chapter 16 - Object-Oriented Programming of the Programming in Lua book.
Lua it's not an OO language "per-se" but offers mechanisms to implement different styles of Object Orientation.
There are a lot of libraries that implement OO for lua. A look at lua.org (the main Lua site) or lua-users (the Lua Community Wiki) will be helpful.
Even more helpful would be to ask in the Lua Mailing list.
It's mostly used as a scripting tool in Apps, to extend or implement functionalities.