There is usually no need for Select or Activate.

You left out the "." for the With. Change:
[VBA] ws.Range("O21:HN494").Select
With Selection
Interior.ColorIndex = xlNone
Borders(xlDiagonalDown).LineStyle = xlNone
Borders(xlDiagonalUp).LineStyle = xlNone
Borders(xlEdgeTop).LineStyle = xlNone
Borders(xlEdgeBottom).LineStyle = xlNone
Borders(xlInsideHorizontal).LineStyle = xlNone
End With [/VBA]
to:
[VBA]
With ws.Range("O21:HN494")
.Interior.ColorIndex = xlNone
.Borders(xlDiagonalDown).LineStyle = xlNone
.Borders(xlDiagonalUp).LineStyle = xlNone
.Borders(xlEdgeTop).LineStyle = xlNone
.Borders(xlEdgeBottom).LineStyle = xlNone
.Borders(xlInsideHorizontal).LineStyle = xlNone
End With [/VBA]