I reseted an iphone. Now when i
switch it on it says insert valid
sim without pin lock on it. Its on
o2. Do i put any sim in it or i
need o2 sim?

if the phone is locked to o2 then you will need an o2 sim card to activate the phone

You could also insert your sim into another phone, disable the pin lock on that phone and then plug it back in to the iPhone if you can't get hold of a valid O2 sim without a pin lock

There is a new build of IOS5.01 which ends in 406. I understand that this is intended to fix/improve this problem. the latest version can be installed by using the restore function on itunes.Check the version on your phone to see if it ends in 405. If is does try the upgrade.

Hopefully you can use this function with an inoperable sim.

That build just fixes a problem with some phones failing to recognise a sim card is present.

The OP's problem is that they cannot activate a phone due to the phone requiring a sim card without a PIN lock on to activate