mysql query bug

That alias is out of scope for the subquery

select id, s
from (
         select o_user_id as id, sum(total_price) as s
         from Orders o
         group by o.o_user_id
     ) as t1
where s = (select max(t1.s) from t1)

You can do

WITH T1 AS 
    (
      select o_user_id as id, sum(total_price) as s
         from Orders o
         group by o.o_user_id
     ) 
SELECT id, s
  FROM T1
 WHERE s = (select max(t1.s) from t1);

If you want only one row, you can use order by and limit:

select o_user_id as id, sum(total_price) as s
from Orders o
group by o.o_user_id
order by s desc
limit 1;

In MySQL 8+, you can use window functions. To get multiple rows in the event of ties, use rank():

select ou.*
from (select o_user_id as id, sum(total_price) as s,
             rank() over (order by sum(total_price) desc) as seqnum
      from Orders o
      group by o.o_user_id
     ) ou
where seqnum = 1;