You should have reported it lost. O2 would have sent a new SIM with the same number. Until reported you are liable for any charges the finder racks up.
Guide: Lost or Stolen O2 Device? - How to Report it
Guide: Cancelling Your Contract
Personally, knowing O2 on cancelling I would get the replacement SIM first
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
