DDL locks

No way to specify DDL with wait option. It will always fail with "resource busy" error.
In 11g there's a way to do this, but before have to use brute force

declare
success boolean := false;
begin
while ( not success)
loop
begin
execute immediate 'alter table t1 drop column n5 ';
success := true;
exception
when others then
null;
dbms_lock.sleep(10);
end;
end loop;
end;


ref: