Structs are a natural way to organize data, but that doesn't necessarily make it "OO". In Lua, a module is typically a table (associative array really) of functions. All of the standard modules that come with Lua are just collections of functions that don't have a concept of "this" (like math or os).

Now, an object in Lua can have functions that assume a "this" parameter (Lua calls it "self") and thus, it can be viewed as an object qua OO, for example, an open file:

    file = io.open("foo.txt","r")
    file:setvbuf('no') -- no buffering
    file:seek('set',10) -- seek to byte 10 in file
    data = file:read(8) -- read 8 bytes
    file:close()        -- close file

but Lua lacks typical OO features like inheritance, a "class" function or method, which are what the OO modules in Lua always add. It's this mentality, that OO is missing if you don't have inheritance.