My main OO collection of steam, diesel and electric locos plus matching stock mainly consists of the following:
Locos and units named after yours truly, places and relatives, where my family and I have lived over the years;
Steam locos that are now preserved that were allocated to depots where my family and I lived over the years with matching pre-nationalisation carriages chosen only from those types that are represented in preservation;
Some models of prototypes built in Birmingham where I was born (mostly DMUs);
Depending on the amount of room at my disposal and the availability of models, I model in both OO and N, and arrange things so collections in both scales complement each other.