There is no contract for you to download. If you ordered online you would have had all the information via email
Everything can be found in MY O2. http://www.o2.co.uk/myo2
Also all terms and conditions for contracts can be found here https://www.o2.co.uk/termsandconditions/mobile
Hope that helps and welcome to the forum
Veritas Numquam Perit
I need my contract - where can I download it?
In your MyO2 (link provided by @Cleoriff above), if you navigate to the "View My Bill" "Download and Print current bill", You'll actually get to download the documents relating explicitly to your contract (as signed on-line or in-store) under "Documents", like the sample below:
Is that what you are after?