change format of part of column

Hi, I'm trying to make my code change the format of 9 columns. they all have the same amount of rows but these may differ each time the macro runs. I tried the following code with an array, but it doesnt work at all. Can you please help me to adjust it 'till it works? and maybe tell me what I did wrong?

Thanks in advance.

Function Format_all()
Dim wsO as worksheet
Dim f As Long
Dim format() As String

For f = 1 To 9
Set format(1) = wsO.Range("A16:A" & wsO.Range("A16").End(xlDown).Row)
Set format(2) = wsO.Range("B16:B" & wsO.Range("B16").End(xlDown).Row)
Set format(3) = wsO.Range("C16:C" & wsO.Range("C16").End(xlDown).Row)
Set format(4) = wsO.Range("D16:D" & wsO.Range("D16").End(xlDown).Row)
Set format(5) = wsO.Range("E16:E" & wsO.Range("E16").End(xlDown).Row)
Set format(6) = wsO.Range("F16:F" & wsO.Range("F16").End(xlDown).Row)
Set format(7) = wsO.Range("G16:G" & wsO.Range("G16").End(xlDown).Row)
Set format(8) = wsO.Range("H16:H" & wsO.Range("H16").End(xlDown).Row)
Set format(9) = wsO.Range("I16:I" & wsO.Range("I16").End(xlDown).Row)

With format(f).Borders(xlEdgeLeft)
        .LineStyle = xlContinuous
        .ColorIndex = 0
        .TintAndShade = 0
        .Weight = xlMedium
    End With
    With format(f).Borders(xlEdgeTop)
        .LineStyle = xlContinuous
        .ColorIndex = 0
        .TintAndShade = 0
        .Weight = xlMedium
    End With
    With format(f).Borders(xlEdgeBottom)
        .LineStyle = xlContinuous
        .ColorIndex = 0
        .TintAndShade = 0
        .Weight = xlMedium
    End With
    With format(f).Borders(xlEdgeRight)
        .LineStyle = xlContinuous
        .ColorIndex = 0
        .TintAndShade = 0
        .Weight = xlMedium
    End With
    Next f
end function

Also, this is all the code I use for this function so nothing else in "option explicit"