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It depends if you upgraded to an eligible tariff.
Check your "extras" in your My O2 http://www.o2.co.uk/myo2
I upgraded an hour ago and got the Disney+ offer. I got 6 text messages and 5 emails with various things about the upgrade and the link given said that i could also redeem in Extras on My O2 which is the way I did it. I was on the laptop, not the app.
Track usage, upgrade and get support with the My O2 appDownload My O2