When booting you'll first see the BeOS Partition Manager. Press enter to start booting from your new partition.
Press space at the boot screen again and select your fail-safe video mode. BeOS doesn't recognize the video card that VirtualBox provides so we have to specify a VESA resolution this way. In a bit we'll make it permanent.
If you select Continue Booting and then get a kernel panic, try going back to the VM settings and completely disabling USB and/or audio. That seemed to do the trick for me but the panic wasn't consistent and sometimes things would work fine with those enabled.
There are a few things we can do to make life easier, first let's set a default video mode so we don't have to select one every time we boot.
Let's try out some features of BeOS. Close the Read Me First window and then on the desktop press Alt+F to bring up Find. Remember earlier when we said Be's filesystem was really advanced? Type 'vesa' and press Search.
Wow, that was fast! That sort of search speed seems like a normal thing today, but remember that this was in the mid-90s. Searching on any other operating system was a very slow task and Be's filesystem was years ahead of its time.
Double click the path for the second entry, /boot/home/config/.../drivers/sample/. That'll open the folder rather than opening the vesa file.
Now push Alt+F again and search for 'drivers'. We want the one in /boot/home/config/settings/kernel/. This time double-click the folder on the left side to open the drivers folder.
The goal is to have these two folders open:
Click and drag to select kernel and vesa, then right-click and drag them into the drivers folder and select Copy Here.
Now close the sample folder. We need to edit both of the new files, so double-click on vesa first and we'll set up our default video mode.
Any of the modes from the boot menu will work, for this I was using 1024 768 16. Add a new line in the same format shown in the comment:
Go to File, Save, and then close this window. Now double-click on kernel and we'll fix a cache bug.
Under the commented line that has disk_cache_size 2048, add a new line: disk_cache_size 16384
Save that file, close it, then go to the BeOS menu in the top right corner and click Restart. This time just let the system boot on its own and it should be in the video mode you entered.
Now you can explore! Check out the demos and applications and take a look at the resource page for some links to tons of BeOS software.
If you're using VirtualBox in macOS be careful of apps that use a lot of resources, like the GLTeapot demo. On my system they seem to take over all of the VM resources and make it unusable, I had to force-quit the VM. On Windows everything seems to work fine.