--# obj is the OO table. do not define functions in here, they must be defined after this declaration for the __newindex metamethod to be invoked

local obj = setmetatable({}, {
  __newindex = function(self, key, value)
    if type(value) == "function" then
      local func = setmetatable({}, {
        __call = function(_, ...)
          if rawequal(self, ({...})[1]) then
            return value(...)
          end
          return value(self, ...)
        end;
      })
      rawset(self, key, func)
    else
      rawset(self, key, value)
    end
  end;
})

--# so add functions to your object here, define them like detailed below

function obj:foo()
  print(self)
end

function obj.bar(self)
  print(self)
end

--# from now on the above functions can be invoked in one of three ways

obj:foo()
obj.foo(obj)
obj.foo() --# this is not normal OO, yet it still works due to the metatable magic.