>This works, but I can't work-out why. Shouldn't oSheet.UsedRange.Rows.Count by itself suffice?

Alan,
The description of UsedRange says that "The UsedRange property represents the area described by the farthest upper-left and farthest lower-right nonempty cells in a worksheet and includes all cells in between." So oSheet.UsedRange.Rows.Count will not be sufficient if you have an empty cells at the begining of sheet.