Can I use Oracle Standard or Enterprise edition for my own training purposes?
Yes, that's fine:


  All software downloads are free, and most come with a Developer License that allows you to use full versions of the products at no charge while developing and prototyping your applications, or for strictly self-educational purposes.


The full Developer License can also be found on OTN, including:


  Oracle grants You a nonexclusive, nontransferable, limited license to internally use the Programs, subject to the restrictions stated in this Agreement, only for the purpose of developing, testing, prototyping, and demonstrating Your application and only as long as Your application has not been used for any data processing, business, commercial, or production purposes, and not for any other purpose.


As with any licensing question, I'd suggest you read Oracle's info carefully including the full license and satisfy yourself that what you want to do is OK rather than just taking my word for it.