create table mytable (
col1 number primary key,
col2 number,
col3 number not null
);

table MYTABLE created.

select table_name, column_name, nullable 
from user_tab_cols where table_name = 'MYTABLE';

TABLE_NAME                     COLUMN_NAME                    NULLABLE
------------------------------ ------------------------------ --------
MYTABLE                        COL1                           N        
MYTABLE                        COL2                           Y        
MYTABLE                        COL3                           N

因此，不，您不需要将主键列指定为NOT NULL。