Are Ordnance Survey (OS) Maps Online Available in the United States?

No. Ordnance Survey (OS) is the national mapping agency of Great Britain (UK). Its digital products — OS Maps Online (web/app), OS Maps API, and premium topo layers — cover only the United Kingdom (England, Scotland, Wales).

There is no OS Maps service for the United States or any other country.

US Equivalents for Topographic/Outdoor Mapping

• USGS Topo Maps (free): topoView, National Map Downloader — official USGS 7.5-min quads (PDF, GeoTIFF).
• Gaia GPS / onX / CalTopo / AllTrails: Popular subscription apps with USGS, Forest Service, and proprietary layers for hiking, hunting, off-roading.
• Google Maps / Google Earth: Terrain layer, but not true topo contours.
• OpenTopoMap / OpenStreetMap: Free, global, community-sourced contour lines.

If you need UK mapping, OS Maps is excellent. For the US, use USGS or the outdoor apps above.