Technically possible: Yes
In practice: No.
When you boot virtual box you will have different hardware than when you boot from the bare metal. This will confuse windows, which tend to store its hardware configuration so it can boot faster.
(Explicitly mentioning windows since other operating systems like FreeBSD or Linux do not do this and it would work for them. The price for that is a slower boot time)
You might get around that by assigning a real partition to contain the disk information in virtual box and always running sysprep before switching to new hardware. However this is extremely bothersome and basically comes down to repeating part of the windows installation whenever you switch between virtualized hardware and the bare metal.
A second way around that it by using VT-D and assigning real hardware to the OS in the VM. That requires a CPU with VT-D (or its AMD equivalent) and extra hardware. (e.g. one graphics card to boot your old OS, one to show in both real win10 and in the virtual box. That might be done as a theoretical exercise, but doing this for real means you basically have the hardware for two PCs.