We get a number of questions about how discounts and data promotions
are applied to O2 tariffs and why it appears not to be added to the account.
In order to help, I've created a small guide.
First thing to note is that your MyO2 http://www.o2.co.uk/myo2 will always display the full tariff price before any discounts are applied. This is what most customers see and assume they aren't getting any discount. (You will see the discount when your bill is generated. Until then it will always show the full price.)
O2 apply your discount as a bolt on (yes I know it sounds backwards) but this bolt on actually credits your account with your applicable discount.
When your bill is generated, the discount is deducted from the tariff price shown in your MyO2, so if you have a tariff of £18 and a bolt on for a £3 discount, your bill will be £15 etc.