No the OPEN and WRITEFILE do not support overwriting the file. You would need to check if the file exists and remove it and create a new one.

Below is an example of checking for a folder and creating it if it does not exist and then writing a new file if it does not exist. If the file does exist it will append to it.

 Code:
Break On
Dim $SO,$Pause
$SO=SetOption('Explicit','On')
$SO=SetOption('NoVarsInStrings','On')
$SO=SetOption('WrapAtEOL','On')

Dim $Folder, $BackupLog
$Folder='C:\UTILS\'
;Check if the folder exists, if not create it otherwise continue on.
If GetFileAttr($Folder) & 16
  ;Folder found do nothing
Else
  MD $Folder
  If @ERROR
    Quit @ERROR
  EndIf
EndIf

$BackupLog = CreateBackupFileLog('C:\UTILS\PSTBACKUPS.TXT','hello')
If @ERROR 
  'Error creating file: ' + @ERROR + ' - ' + @SERROR ?
EndIf

Function CreateBackupFileLog($PathAndFile,$Data)
  Dim $Handle,$OpenFile,$WL,$CloseFile
  If Not Exist($PathAndFile)
    $Handle = FreeFileHandle()
    If $Handle > 0
      $OpenFile = Open($Handle,$PathAndFile,5)
      $WL = WriteLine($Handle, $Data)
      $CloseFile = Close($Handle)
    EndIf
  EndIf
EndFunction