When did you fill out the form?
Timescales are as follows (taken from https://www.o2.co.uk/help/device-and-sim-support/how-to-keep-your-mobile-number😞
"You can give us your PAC code when you order your new O2 connection or at any time afterwards. Once you’ve given us your PAC code, you’ll switch to our network within one working day (Monday-Friday, 9am-5pm, excluding bank holidays). If we receive your code after 5pm, you’ll switch within two working days. If you provide your PAC code at the weekend, you’ll switch on the following Tuesday."
Going by the above, if you gave them your PAC over the weekend or today, your number will transfer tomorrow (Tuesday).
If you've waited longer, you'll need to contact O2.
