I want to use a Watson Assistant search skill to search an Oracle Database, I don't know if the search skill is limited to just these and whether I can use any of these for my search?
Box Salesforce Microsoft SharePoint Online Microsoft SharePoint OnPrem Web Crawl IBM Cloud Object Storage
As you might have seen, the IBM Watson Assistant search skill is based on reaching out to Watson Discovery on IBM Cloud. That cloud service only supports the data source listed in your question.
If you deploy Watson Assistant and Watson Discovery yourself, there are more options available, including Oracle access via JDBC.