I guess the process would depend on whether it was Council approval and/or OC approval that required. I will assume that it was only OC approval that was required.
You mentioned that only part of a wall was removed between the kitchen and the lounge room. Was this a load bearing wall? If not then it would probably be classified as only a Minor Renovation as it is not a structural change. Minor Renovations include reconfiguration of walls.
You would need to put a Motion on the Agenda that requires approval by Ordinary Resolution. The Motion would need to state what works have been carried out e.g. creating an opening in a non-load bearing wall between the kitchen and lounge room.
To strengthen your request for approval you could obtain a report from a builder that states that (a) the work has been carried out competently, (b) that no load bearing walls were changed, and (c) that the renovation has not caused any damage to the common property.
In your Explanatory Material that would accompany your Motion you could state that you were not aware that the works had not been approved, and that you do not know which builder undertook the works therefore you are obtaining a building report of the completed works, and that you are seeking retrospective approval of this works by the OC.
It also may strengthen your case if you aware that other Owners in your building have already obtained approval to undertake this type of work. A conversation with neighbours or a search of the strata records should reveal this information.
Owners Corporations must act reasonably when approving requests for renovations.