ADO is one of the primary MDAC technologies (the two others being OLE DB and ODBC). In fact, in order to use ADO, MDAC needs to be installed.
ADO is a wrapper around OLE DB that privdes a consistent interface to the (various) OLE DB providers. Thus, ADO is used by client applications to access and manipulate data from various sources through OLE-DB.
Although conceptually, ADO is not tied to OLE-DB, the only engine that supports the ADO model is OLE-DB.
Adding reference to VBA project
A reference to the ADO library (Microsoft ActiveX Data Objects 6.1 Library) in an Excel VBA project can be made in the immediate window: