What is the purpose/content of the training ? For self-learning purposes, all of the database versions available at Oracle Downloads | Software | Oracle are "free". Currently, no Oracle database software is certified/supported on CentOS, although folks have managed to install it on this OS, with the assumption that they will have to troubleshoot any issues on their own. Again, for self-learning purposes, Oracle Linux is freely available at https://linux.oracle.com/pls/apex/f?p=101:3
If it is used for commercially training people ( i.e training for profit) then it is probably not free use and hence you should not install any oracle products without a license.