This is a limitation in Oracle Spatial 8i. The length was increased to 32 characters at Oracle 9.0.1. See the Oracle Spatial 9i User Guide and Reference (Preface: New features).
Oracle 9i is not supported with ArcSDE 8.1 Service Pack 2 and earlier.
If you are going to register your table as versioned with the geodatabase, you must limit the spatial index name to two characters shorter than the maximum length (16 or 30). When ArcSDE versions a business table, it creates an ADDs table. If the business table has an Oracle Spatial spatial index, a spatial index is created for the ADDS table as well. The name of this new index is the the name of the existing index with the _A characters appended to it.