SRID

You can view the SRID information in existing SDO_GEOMETRY columns with the following

SELECT g.GEOMETRY.SDO_SRID FROM MY_TABLE g;

assuming that MY_TABLE has a sdo_geometry column called geometry.

Well, what is the srid?

SRID stands for Spatial Reference Identifier.

Without setting a SRID all values in Oracle spatial are treated as Cartesian.
Using the UNIT parameter will never be supported (for SDO_NN, SDO_WITHIN_DISTANCE, BUFFER, LENGTH, DISTANCE, AREA, etc).
Coordinate system transformations will never be supported, so you won't be able to use a query window or compare to a geometry that has an SRID set.
Most/all tools support spatial data with a NULL SRID.

The SRID allows the correct projection, transformation and relational operations between different co-ordinate systems. The most commonly used SRID in Oracle Spatial is 8307 which is the whole earth projection WGS84 using latitude and longitude.

The complete list of Oracle SRIDs can be found in the MDSYS.CS_SRS table.

Behind Oracle Spatial

Oracle Spatial is installed under the MDSYS user account.

MDSYS is a high privilege account with rights similar to the system accounts. The user account is locked by default on more recent versions of Oracle. If your MDSYS user account is unlocked, lock it immediately, you should never need to login as MDSYS, even to create data.

MDSYS has rights with admin option so it can in turn grant rights to other users.
MDSYS is the owner of all the spatial objects, types, metadata, functions, packages and procedures.

If you have installed locator then the MDSYS schema will contain fewer objects than the full spatial option.

Remember, spatial is a licenceable option. Contact Oracle for pricing in your region.