The equivalent is Nz.
For example
Nz(x, "Unknown)
in Access and
NVL(x, 'Unknown')
in Oracle will both return the string "Unknown" if x is null. Otherwise,
they will return the value of x.
Note that Nz is only available within Access itself. It is not available if
you are accessing an Access database from outside Access (for example from
VB via ADO, DAO or ODBC). In that case you can use IIf, as in:
IIf(IsNull(x),"Unknown",x)