O-scale tool kits always call for a hammer, first and foremost; it's not called the "Blacksmith's scale" for nothing.
But seriously, folks - I have seen kits marketed by Micro-Mark as "model railroading tool kits", but I think they had modeling in mind, and you seem to have a maintenance goal.
I don't know of any, as such. Basic medium to small hand tools are all you need; there are really no specialized tools, except for wheel/gear pullers and coupler riveting tools and the like. "Changing batteries" is like, well, changing a battery. Open it up, put in a new one. Traction tires require tweezers, small screwdriver or the like, small wrenches for steam side rod screws.
Check out Micro-Mark for small tools; search the Web for "small tools", and so forth - but, most of the "small" tools you will need for O-scale can be found at the hardware store.
A medium and small hammers really are good things to have, BTW. Sometimes little things need a friendly "tap".