Positioning of imported layers

[SarahStapley]

Hello

I am using ArcGIS online for the first time.  I have tried to import a zipped shapefile of polygons based on OS mapping but notice that the imported layer is some meters offset to the ESRI baselayer I have chose to use.

Can you help me understand why this is please and how I can fix it? 

Thanks

Sarah

[MikeMinami]

What coordinate system is your data currently stored in? I'd suggest transforming it to Web Mercator Auxillary Sphere first.

Thanks,

Mike