1. It is possible that a maximally great being exists.2. If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.3. If a maximally great being exists in some possible world, then it exists in every possible world.4. If a maximally great being exists in every possible world, then it exists in the actual world.5. If a maximally great being exists in the actual world, then a maximally great being exists.6. Therefore, a maximally great being exists.
Obiously something is wrong in there but it's hard to identify exactly what!
The problem is that defing god as something 'maximally great' incidentally defines god as necessarily existing because 'neccessary existence' is greater than mere 'existence'. While that is acceptable as a definition, it is problematic to use it as a premise in the OA because it assumes what it sets out to prove.
For purposes of the OA you have to define a god as a entity that is 'maximally great, except it it does not necessarily exist', that is we still allow gods to possibly exist but we remove the hidden assumption their neccessary, certain existence.
Of course that means the OA no longer works.