What is "SAP on Oracle"
SAP AG (Often just called SAP) is a company which has a product called SAP ERP (Enterprise Resource Planning).  Since Oracle also makes ERP software they are indeed competitors.  In fact they are the top two ERP vendors.  

SAP can be used with quite a few different database systems, but according to one source I found Oracle is dominant comprising around 80% of SAP systems.  Oracle recently won a law suit against SAP for intellectual-property theft. (http://news.cnet.com/8301-1001_3-20029601-92.html#ixzz1CWy7UiKR) 

Here is some SAP on Oracle documentation.

It does seem like a strange scenario for a product to depend so heavily on a competitors product, but it is not unheard of.  To some extent this pattern exists for Oracle with their database product running on Microsoft Windows servers.