Strangely enough no such function exists (or at least I couldn’t find it).. PostGIS has ST_StartPoint and ST_EndPoint, SQL Server 2008 Spatial has STStartPoint and STEndPoint but not Oracle. It does however has SDO_LRS.GEOM_SEGMENT_START_PT and SDO_LRS.GEOM_SEGMENT_END_PT but this applies (it seems) only for measured polylines – and you need to have the full Oracle Spatial installed. With versions that only have Locator you are out of luck.
So putting my thrifty hat again, I put together 2 simple functions (with the very imaginative names get_start_point and get_end_point) that do just that and included them in the package. Functions should work with both 2D and 3D data. You can download the update “thrifty” package here (or from the box widget on the right).