The screenshot indicates the client is trying to download the boot file from 192.168.1.1 via TFTP. If that address is not the OSD server, it’s likely that VirtualBox is providing a DHCP service and trying to connect to something else for the OS download.
I haven’t used VirtualBox in a while, but most of the virtual hypervisors provide several different types of interfaces to the virtual machine. With a “NAT” interface, the VM shares a network connection with the host, and the host provides a DHCP address to the virtual client along with a NAT service allowing the client to reach out to the physical network. This is what it looks like your interface is using. A “bridged” connection connects your virtual machine to the physical interface, assigning it a different MAC address, and does not provide an internal DHCP service so your VM gets an address on the public network (upstream DHCP server or a static IP, if you configure your VM that way). A “host-only” interface gives your VM a virtual interface that is disconnected from your physical network entirely. Some supervisors allow more complex virtual switching abilities such as multiple virtual internal networks and routing interfaces to the physical network.
You may need to switch your VM client and VM OSD servers to host-only, bridged, or virtual switch networks to prevent the client from being served by the VirtualBox DHCP service.