To display the OpenStreetMap tiles dynamically (means in a different coordinate system as provided by the tile service provider) a referenced ArcGIS Server map service (local connection) is recommended. If no map service is configured, the default ArcGIS Server map service for selection display is taken (SynMapService by default). Because the rendering of OSM tiles dynamically needs a lot of resources, another map service in the projection of the main map service should be defined to distribute the service load.

ArcGIS Server map service for selection display (Bing) - Maps category
For details on how to configure the ArcGIS Server map service for selection display see chapter ArcGIS Server Map Service for selection display.