OK. The initial question wasn't clear enough...sorry. This might be answering my own question. Using a foreach loop, this seems to be the only way to access properties and data of the sheet.
foreach (var oSheet in oWB.Sheets)
{
Excel._Worksheet sheet = oSheet as Excel._Worksheet;
if(sheet.Name.Contains( $TEST_VAL )
{
function1(sheet);
}
Marshal.ReleaseComObject(sheet);
sheet = null;
}