Create db link:

create public database link <LINK_NAME> 
connect to <USER> identified by <PASS> 
using '<HOST>:<PORT>/<SID>';

Example:

create public database link STAGE 
connect to STAGE identified by STAGE 
using 'aos.su.local:1521/AOS'

Drop db link:

drop public database link <LINK_NAME>;

Example:

drop public database link STAGE;

List all db links:

select * from dba_db_links;