if you want to check table exist or not then query should like below...
select count(*)
from all_objects
where object_type in ('TABLE','VIEW')
and object_name = 'your_table_name';
OR...
DECLARE tbl_exist PLS_INTEGER;
BEGIN
select count(*) into tbl_exist from user_tables where table_name = 'mytable';
if tbl_exist = 1 then
execute immediate 'drop table mytable';
end if;
END;
Note: when table created with double-quote then it's case sensetive name so in that case your_table_name should be in same case
Happy Coding!
:)