I have a small problem.

We have a setup where all "secure" backups are behind another Debian Server on our network and we would like to setup a openvpn server to get access to the backup network so we don't have to physical move the computer to the other network.

So our network is like this from out to in

Internet ---> Our 10.0.2.0/24 network ( Our network ) ---> Debian Server ---> our 10.0.3.0/24 network ( Our Backup network )

Does someone knows some guides to make a openvpn config there can be bridged so we can get access to the backup data securely

You don't need OpenVPN.
Just enable forwarding on the Debian server (with appropriate firewall rules if you need access control) and set the proper routes on your client(s).

Where do you install your OpenVPN service? Is it on Debian server? or Is it one of the boxes at 10.0.2.0/24?