London Tube station info?

Wanting to buy Oyster Cards for our upcoming trip to London, but wondering if I can save the mailing costs by buying at the tube station our train will arrive in. Can anyone tell me what the operating hours are for the ticket windows in the tube? Cheers
Update: *Adding details. We will be arriving by train, but it will be the Stansted Express into the Liverpool Station and probably around 11:30 or later at night.
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