You can get an even cheaper version of the Oyster Card if you’re buying it online before arriving in London. Check this guide (to airport transport from Stansted) where there’s some info about the online version of the oyster card: http://londontravelhacks.com/transport-stansted-airport-london/

If you are eligible for any concessionary discount (child, railcard, &c.), DO NOT use contactless, since you will be charged the full fare. Instead, always obtain an Oyster card and register your entitlement to a discount at a ticket office (National Rail stations in London can usually do this).

It might be worth noting that these days any contactless credit or debit card can be used in place of an Oystercard. You will be charged the same as if it were an Oystercard. However, teenagers may not yet have their own bank account and debit card, and not all cards are contactless.

If you have multiple cards make very sure that you choose one and only one card to travel with. Otherwise you may be charged two penalty fares for touching in with one card and out with a different one!

In January 2016, you could not get a refund within 48 hours.

During our visit to London on the 1st of January 2016, we found out you can not get a refund for your Oyster card through a machine at the station within 48 hours of buying it.
This was confirmed by a Transport for London employee, but I could not find this requirement in writing, now or then.

Note that there is a specific option for a Visitor Oyster card. This is the same as a normal Oyster, but you pay a small fee (£3) to buy it instead of a deposit (£5) on a regular Oyster.

One big advantage of a visitor Oyster is that 11-15 yr olds can get the child discount without needing a Zip photocard. According to https://visitorshop.tfl.gov.uk/help/visitor-oyster-card/ :

Children aged 11 to 15 can also use a Visitor Oyster card and benefit
of the Young Visitor discount – this can be added by a member of the
staff at most Tube stations in London and offer children 11 to 15
years old half adult pay as you go rate for up to 14 days.

On the site of the London Travel card you find the prices of the travel cards and the Oystercard caps for the same zones.

Travelcard Prices
(these are subject to change and prices are controlled by TFL)

Select type Day Anytime Price Off Peak Day Price
Adult Zones 1-2 £12.00 £12.00
Adult Zones 1-4 £12.00 £12.00
Adult Zones 1-6 £17.00 £12.00

Oyster Card Capped Prices

Select type Day Anytime Price Off Peak Day Price
Adult Zones 1-2 £6.40 £6.40
Adult Zones 1-4 £9.20 £9.20
Adult Zones 1-6 £11.70 £11.70

So yes, you are right to want an Oystercard for two days use of a travel card, even if you do not get the deposit refunded.

You pay a £5 refundable deposit to get an Oyster card.
You can get an Oyster card at any tube station as well as at several other locations.

This site explains the refund.

If you don’t have a contactless and Oyster account, you may be able to get a refund from a Tube station ticket machine if your card has:

No more than £10 pay as you go credit
No valid season ticket

You get an Oyster card from the machine at a tube station. As for the refund

Oyster Card Refunds

The £5 deposit you pay for an Oyster card is
refundable along with any pre-pay (Pay as you go) money left on the
card.

Before you claim a refund, it’s worth bearing in mind that if you’ve
used your Oyster card on a Pay as you go basis and it’s not loaded
with a weekly Travelcard or bus pass then it’s transferable. And the
money doesn’t expire so you can give it to a friend, relative or
anyone else to use.

If you don’t know anyone who might want to use an Oyster or you don’t
plan to return to London, you can get back your unused money.

Where to get a refund in London

Your £5 deposit and up to £10 of Pay as you go money can be refunded
at any London Underground (tube) station ticket machine

If you have more than £10 of Pay as you go money refunds are available
at TfL Visitor Centres.

If you have an online account with TfL, refunds are available online.

Once you get your Pay as you go credit and deposit refunded you will
no longer be able to use the Oyster card.

How to get a refund after leaving London

If you can’t visit an underground station before you leave, send your
Oyster card and photocopied proof of name and address to TfL:

Oyster Refunds
4th Floor
14 Pier Walk North
Greenwich
London SE10 0ES

Refunds by post will be sent by cheque (£) or by bank transfer to your
bank accounts (UK addresses only).

If you have any questions, contact TfL Customer Services on 0343 222
1234 (open daily 8am–8pm).

Last updated: 5 January 2016

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