By default, all text tokens are converted to uppercase and then indexed. This results in case-insensitive queries. For example, separate queries on each of the three words cat, CAT, and Cat all return the same documents.
You can change the default and have the index record tokens as they appear in the text. When you create a case-sensitive index, you must specify your queries with exact case to match documents. For example, if a document contains Cat, you must specify your query as Cat to match this document. Specifying cat or CAT does not return the document.
To enable or disable case-sensitive indexing, use the mixed_case attribute of the BASIC_LEXER preference.
Oracle Text Reference to learn more about the BASIC_LEXER