create table t (
  x varchar2(100)
);

insert into t
  select dbms_random.string('a', 10) from dual connect by level <= 10;

select * from t;

X           
rymGBKBuwd  
EKHxpzrDUy  
HIuhcZEATc  
kooiSDxcJx  
IRroyVLfpE  
TNVKETYrGU  
nmjlMXNEaz  
xAVMnGdGMR  
dYNonpIMZZ  
YkPtiUpEsY 

update t
set    x = 'OLD' || x;

select * from t;

X              
OLDrymGBKBuwd  
OLDEKHxpzrDUy  
OLDHIuhcZEATc  
OLDkooiSDxcJx  
OLDIRroyVLfpE  
OLDTNVKETYrGU  
OLDnmjlMXNEaz  
OLDxAVMnGdGMR  
OLDdYNonpIMZZ  
OLDYkPtiUpEsY