I wish to choose a view from the sys user, however with other connection. I have to choose view DBA_Customers. Can there be in whatever way of doing this? I'm new in Oracle, so perhaps it's silly question, but nonetheless, I've no clue how to get it done.

Perhaps you have attempted select * from SYS.DBA_USERS; ? (it's been a very long time since i have used Oracle)
(The consumer you're really hooking up from might not have the rights to see the table.)
Also you could attempt select * from ALL_USERS;