When Oliver and Amelia are in the battle of the department of mysteries, they find themselves in a room. It's the same one that no one is able to unlock.
Oliver was able to get in there because his love for Amelia opened the lock. Once inside he finds a well that is full of Amortentia which is the greatest love potion of all time. In an interview, JK Rowling said this was what was in the locked room. The most dangerous thing of them all. Yet, Oliver could open it and the fact that him and Amelia are in there together is actually pretty significant and we're definitely going to see this play a bigger role in the chapters to come so keep this in the back of your mind whenever you're reading anything from here on out.
Oliver smells chocolate and raspberries because those are his favourite things and the ocean is like from when he and Amelia were on the boat together going home/to Kalishterya and those are his favourite memories.
Honestly Oliver is such a sweetie and writing this last chapter from his point of view was a really awesome thing for me as an author to be able to do because he is probably one of my favourite characters.