The operating system you use will have to be placed on a single Micro SD card. Using a mac the below command was useful to place the downloaded Oracle Linux 7 distribution for the Raspberry Pi on the Micro SD card:
sudo dd bs=1m if=/var/tmp/rpi3-ol7.3-image.img of=/dev/disk2 conv=sync
If you face the issue of the below error, you most likely have mounted the SD card to your operating system. You will have to unmount it (via the disk utility app) and retry the command. Do note this could take some time to complete.
dd: /dev/disk2: Resource busy
A bit of care is needed when executing the command. If your Micro SD card is NOT mounted on /dev/disk2 you might run into the issue that you damage an existing other disk. Meaning, you need to check if /dev/disk2 is indeed the SD card in your case. Using a Mac you can use the below command to check your disks:
When your dd command is finished and you place the SD card in your Raspberry Pi and start it you should end up with a running Oracle Linux 7 operating system on your Raspberry Pi.