In Oracle 12c DEFAULT values for columns can directly refer to Oracle sequences.

• DEFAULT sequence.NEXTVAL
• DEFAULT sequence.CURRVAL

create sequence test_seq;
create synonym test_seq1 for test_seq;

create table test_tbl
(id  number default test_seq.currval,
 id1 number default test_seq.nextval,
 id2 number default test_seq1.currval,
 id3 number);

insert into test_tbl(id3) values(10);
insert into test_tbl(id3) values(20);
insert into test_tbl(id3) values(30);

select * from test_tbl;
        ID        ID1        ID2        ID3
---------- ---------- ---------- ----------
         1          1          1         10 
         2          2          2         20 
         3          3          3         30

Still you can insert NULL and explicit values for such columns

insert into test_tbl values(default, NULL, 5, 40);

select * from test_tbl where id3=40;

        ID        ID1        ID2        ID3
---------- ---------- ---------- ----------
         3                     5         40

Facts to remember

• If sequence is not fully qualified by specifying the sequence owner then sequence owner is the user who issues the CREATE TABLE statement
• Synonyms on sequences undergo a full name resolution and are stored as the fully qualified sequence in the data dictionary. It is true for public and private synonyms

declare
  v_id number;
begin  
 v_id := dbms_metadata.session_transform;

 dbms_metadata.set_transform_param(v_id, 'STORAGE', FALSE);
 dbms_metadata.set_transform_param(v_id, 'SEGMENT_ATTRIBUTES', FALSE);
end;
/

select dbms_metadata.get_ddl('TABLE', 'TEST_TBL') from dual;

  CREATE TABLE "TOMASZ"."TEST_TBL" 
   (    "ID" NUMBER DEFAULT "TOMASZ"."TEST_SEQ"."CURRVAL", 
    "ID1" NUMBER DEFAULT "TOMASZ"."TEST_SEQ"."NEXTVAL", 
    "ID2" NUMBER DEFAULT "TOMASZ"."TEST_SEQ"."CURRVAL", 
    "ID3" NUMBER
   )

Have a fun 🙂

Tomasz