The OpenPanel team doesn’t support Ubuntu 12.04 server officially yet, but I got it installed with a few alternate steps and it seems to work fine. I have it running in a production environment.

The original installation steps for older Ubuntu versions are described at http://www.openpanel.com/download/openpanel-download/.
But for Ubuntu 12.04 just follow the steps below and it will work. Make sure you execute all steps as root.

nano /etc/apt/sources.list

Add the following lines at the very bottom of /etc/apt/sources.list, save it and go back to the terminal with Ctrl + X and than Y for the confirmation question.

deb http://download.openpanel.com/deb/ precise main
deb-src http://download.openpanel.com/deb/ precise main

Execute the following commands:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4EAC69B9
apt-get update

You will probably get a warning when everything is updated about a distribution conflict: “W: Conflicting distribution: http://download.openpanel.com precise InRelease (expected precise but got natty)”, just ignore it. (This warning should be fixable with: http://forum.openpanel.com/viewtopic.php?f=2&t=602&hilit=precise, but without, it will also work)

Remove no longer required packages:

apt-get -f install
apt-get autoremove

Install libzip:

wget https://launchpad.net/ubuntu/+archive/primary/+files/libzip1_0.9.3-1_amd64.deb
sudo dpkg -i libzip1_0.9.3-1_amd64.deb

Install libmysqlclient16, you will probably get an error but just ignore it. It will be used further on.

wget http://launchpadlibrarian.net/94808408/libmysqlclient16_5.1.58-1ubuntu5_amd64.deb
sudo dpkg -i libmysqlclient16_5.1.58-1ubuntu5_amd64.deb

Start the actual installation of OpenPanel:

aptitude install openpanel-suggested

I got this question before it would continue the install and answered yes, which will install the apache2-mpm-prefork:

The following packages have unmet dependencies:
 apache2-mpm-worker : Conflicts: apache2-mpm which is a virtual package.
 apache2-mpm-prefork : Conflicts: apache2-mpm which is a virtual package.
The following actions will resolve these dependencies:

     Keep the following packages at their current version:
1)     apache2-mpm-worker [Not Installed]                 

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