When you say “includes OSM viewer” what exactly do you mean? OpenStreetMap is just data; there are many different ways of showing that data to users (“renderers”). What are you doing in your app that allows people to show things based on OSM Data?
People asking questions such as yours sometimes think that the “standard” map tiles that they see at https://www.openstreetmap.org/ are OpenStreetMap and the only choice that they have to make is “those map tiles, or some similar ones in a different style”. That’s not really true - for example, if I go to https://www.bing.com/maps/ in the UK where I live and zoom in I see data that claims to be derived from both HERE and OSM, and the map display there doesn’t use the same sort of map tiles at all.
If you do decide to use an online resource for OSM data, then the chances are that that will have some sort of terms of use associated with it. For example, the terms of use associated with OpenStreetMap’s “standard” raster map tiles can be found at
https://operations.osmfoundation.org/policies/tiles/ .
