from http://msdn.microsoft.com/en-us/library/bb820957(office.12).aspx you will find that outlook save the last update time of an OAB in the Windows registry: HKCU\Software\Microsoft\Exchange\Exchange Provider\OAB Last Modified Time. but The type of this registry entry is REG_BINARY. The data is 8 bytes in size. You can convert the data to a 64-bit FILETIME structure specifying a Universal Coordinated Time (UTC) value that Outlook last downloaded the OAB files from the Exchange server to the client computer.

i am using outlook 2010, so there will be more than 1 entry.

here is an ps script:

Get-ItemProperty "hkcu:\Software\Microsoft\exchange\exchange provider\OABs\*" | foreach {
$byteDate = $_."OAB Last Modified Time"
$int64Date = 
[System.BitConverter]::ToInt64($byteDate,0)
$tempDate = 
[datetime]::FromBinary($int64Date)
$tempDate.AddYears(1600)
}

jeff provided a VBS script to help you change the value as an easy read type at http://www.expta.com/2010/06/vbscript-to-tell-last-time-oab-was.html.

hope helps.

thanks GuHuajun.