Run:
qemu-system-x86_64 --enable-kvm -cpu host -m 2G -drive file=oracle-qemu.qcow2
If you don't have kvm, remove --enable-kvm and -cpu host. Note that not having kvm will make it very slow.
The default passwords are:
test: test
oracle: Oracle123
SYS, SYSTEM: Oracle123 (inside oracle)
To use sqlplus, login as oracle, and from within bash, run:
sqlplus sys/Oracle123 as sysdba