I'm trying to run LineageOS in VirtualBox. I have already downloaded the ZIP-file and tried to create a bootable ISO-file of the content with mkisofs. This was not successful because VirtualBox could not find a bootable medium (before booting I added the ISO to the optical drive).
In your comment, you say
I know that LineageOS is not build for the PC but I want to try it out without the need of flashing it to my phone. It is possible to run the original Android image in VirtualBox although it is not build for this purpose.
As you point out, LineageOS is not built for the Intel x86 architecture used in PC processors, but for the ARM architecture used in most mobile devices.
When you say the "original Android image" can be run in VirtualBox, perhaps you're thinking of the Android x86 project? This is a version of Android that has been modified to run on x86 processors. Similarly, in order to run on x86 processors (and in VirtualBox) someone would need to port the LineageOS code itself and create a compatible build. You won't be able to run the OnePlus 3 version in VirtualBox as it is built for ARM processors.
If you want to try it out without flashing it to your phone, perhaps you could use some sort of Android Device Emulator for the PC?
Actually, there IS a LineageOS build for x86 PCs - including emulators (such as vmWare and VirtualBox). You are thinking (like all too many) that x86 is not a valid target - despite x86 being not only a valid generic target, but a Google-supported target - going all the way back to Lollipop. What has been lacking is NOT the support in Android - but in the generic x86-emulation support community (neither vmWare or Oracle VirtualBox supports Android as a guest); you have to make VirtualBox (for example) think that Android is Linux to get it to run at all. Alternatively, you can take that same ISO and run it on a USB stick (that is something that I personally have done since the days of Lollipop-x86).