You can't use bitcoin-qt for an offline wallet. I suggest electrum. Here are the steps for debian but I imagine they will be the same for ubuntu:
sudo apt-get update
sudo apt-get install python-pip python-qt4
wget https://download.electrum.org/Electrum-1.9.7.tar.gz #software
wget https://download.electrum.org/Electrum-1.9.7.tar.gz.asc #signature
wget https://github.com/spesmilo/electrum/raw/master/pubkeys/ThomasV.asc #developers' gpg public keys
Then run gpg once to make it generate the default key pairs and get that over with. Press control c to exit gpg and do this:
gpg --import ThomasV.asc
gpg --verify Electrum-1.9.7.tar.gz.asc Electrum-1.9.7.tar.gz
It should now tell you "good signature" for ThomasV. There will be a warning which you can safely ignore.
gpg: Good signature from "ThomasV <>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 6694 D8DE 7BE8 EE56 31BE D950 2BD5 824B 7F94 70E6
Then install electrum:
sudo pip install Electrum-1.9.7.tar.gz
Now you can disconnect from the internets.
Then run it (just type electrum) and it will create a new wallet for you. Note down the seed and there's your wallet! To create a watch only wallet copy the master public key over to a USB drive for use on your regular OS install:
https://electrum.org/tutorials.html#offline-mpk