Tristan Miller wrote:

Does anyone know if/where it is safe to punch a hole through an Oyster
Card? Ideally I'd like to punch the hole near one of the corners, or near
one of the short edges.

I'd strongly advise not to, both for the reasons mentioned in this
thread and because you'd be in breach of TfL's Conditions of Carriage by
doing so, specifically Condition 5.1:

All photocards and Oyster photocards remain our property and must not be
intentionally damaged, altered or tampered with in any way. We may
withdraw or cancel your photocard or Oyster photocard at any time. We
will only do this for a good reason and if we do, we will give you a
receipt. If a photocard is needed, you can only use your ticket or
Oyster card when you have your photocard with you. This also applies to
Oyster photocards when used with a printed ticket.

The Conditions of Carriage can be found at
http://www.tfl.gov.uk/termsandconditions/899.aspx

The equivalent in the National Rail Conditions of Carriage is Condition 23:

If a ticket has been damaged or has been tampered with or altered in any
way, it is not valid for travel. However, if you return it to the Train
Company or travel agent which sold it to you, they will arrange for a
replacement ticket to be issued unless it has reason to suspect that the
ticket has, or will be, used for fraudulent or improper purposes. You
may have to pay a reasonable administrative charge (not exceeding £10)
for the replacement.

And those can be found at http://www.nationalrail.co.uk/times_fares/nrcc/

HTH,

Barry