virtualization

OS X is licensed for use only on Apple-branded hardware, and different versions have different restrictions around what's permissible and what's not within a virtual machine; visit the Apple legal and licensing web site for details.

The details of creating a bootable image vary by version; there are directions for creating an installer disk from the Mavericks download posted around the 'net, including some directions posted at Apple.

(I'll leave the discussion of the differences between criminal and civil law for another time.)

It is only legal to run OS X in a virtual machine if the host computer is a Mac. Therefore yes it would be legal to run OS X in VirtualBox if VirtualBox is running on a Mac. The same would apply to VMware Fusion and Parallels.

While VMware Fusion and Parallels implement checks and will only allow an OS X guest if the host is a real Mac I don't believe VirtualBox does this. However it is also the case that VMware Fusion and Parallels have built-in support for OS X guests to make it much easier to install and setup an OS X guest it is therefore not necessary to make a special OS X iso image.

VirtualBox has no built-in support for OS X guests even on a real Mac host and as a result even if you did make an iso image of OS X which is possible, by itself it will not work. It is hypothetically possible to get round this problem but does require more expertise and additional tools.

In this scenario if you used VMware Fusion or Parallels or VirtualBox they would all allow running an OS X guest in a virtual machine on a Mac running OS X as the host. It is also possible and legal to run OS X as a guest in VMware ESXi but again only if you are using a real Mac. If you use ESXi then the Mac will boot in to the ESXi hypervisor and run OS X as a guest. It is currently recommended to use ESXi 5.5 and virtual machines of version 8 or 9. Apparently the web client to manage VSphere/ESXi does not support version 10 virtual machines and you need to use the Windows only client.