Change the properties of an existing table.
  
  Syntax:
Change the name, physical and logical storage parameters of an existing table. Enable or disable Constraints, Triggers and Primary Keys:
ALTER TABLE properties
Add, modify or drop columns:
ALTER TABLE column_properties
Change the Constraints and Primary Key for an existing table:
ALTER TABLE constraints
Add, modify or drop columns from a Partitioned table:
ALTER TABLE partitioning_clause
Add, modify or drop columns from an external table:
ALTER TABLE external_clause
Example
Add a column to a table: ALTER TABLE STAFF_OPTIONS ADD SO_INSURANCE_PROVIDER Varchar2(35);
For more examples - follow the links above
"At a good table we may go to school" ~ Thomas Fuller
  
  Related Oracle Commands:
  
  ANALYZE TABLE COMPUTE STATISTICS
  ALTER INDEX
  ALTER VIEW
  COMMENT - Add a comment to a table or a column. 
  RENAME 
  DBMS_REDEFINITION 
  
Related Views:
  DBA_ALL_TABLES       ALL_ALL_TABLES       USER_ALL_TABLES
  DBA_TABLES           ALL_TABLES           USER_TABLES         TAB
  DBA_TAB_COLUMNS      ALL_TAB_COLUMNS      USER_TAB_COLUMNS 
  DBA_PART_TABLES      ALL_PART_TABLES      USER_PART_TABLES  
                                                                DICTIONARY
                                                                DICT_COLUMNS