The TITANIC and Olympic were almost identical and some photographs of the TITANIC are actually the Olympic which was TITANIC's sister-ship.
1. One difference is the A-Deck. On the Olympic, A-Deck was all open. On the TITANIC, A-Deck was half enclosed and half open.
2. This may not be distinguishable from photographs, but the TITANIC was larger than the Olympic.
3. Most interiors are the same but one of the things that the TITANIC had that the Olympic didn't was a Reading and Writing Room.