How do i get list of country, state and city which supports OSM

How do i get list of country, state and city which supports OSM I want to populate the drop downs and on selecting those need to generate a marker on map. I have web interface which has 3 drop downs country, state and city. Data is taken from the freely available on internet. While I selects the data from the drop down and click on map it for the OSM (here i used java script API of OSM) many of the locations are not able to display on the map. Hence i am searching for the specific countries, states and city database supported by OSM.