Hi Mark,
Are you looking to reference the Tiled Service from you ArcGIS Server instance? If so, you can do this by going to My Content > Add Item > From the Web > enter the URL to the tiled service.
If you are looking to have ArcGIS Online host the tiled service you can export the cache out to a TPK file using the Export Map Server Cache tool. You will specify 'Tile Package' for the Export Cache Type parameter.
Next upload the TPK to AGOL using the Share Package tool. Finally, you will see an option to Publish the TPK in AGOL.
Also, here is a document that discusses similar steps:
https://community.esri.com/docs/DOC-2996