To create a primary key

ALTER TABLE <table_name> ADD CONSTRAINT <primary_key_constraint_name> PRIMARY KEY (<column_name>) USING INDEX

To drop a primary key:

ALTER TABLE <tablename> DROP PRIMARY KEY

To view all the indexed columns includin the primarky key columns:

SELECT * FROM user_ind_columns WHERE table_name='<tablename>' ORDER BY index_name, column_position

To view the constraint names on a table (including the primary key name):

SELECT * FROM user_constraints WHERE table_name = '<tablename>'

To view the constraint columns and constraint names on a table (including the primary key):

SELECT * FROM user_cons_columns WHERE table_name = '<tablename>'

Good oracle reference:
http://www.psoug.org/reference/constraints.html