Oracle's left join and where clauses errors
When you add WHERE filters to the optional/outer table, then you change the query into an INNER JOIN. You need to add the condition into the join, or derived table, or CTE.

select ATABLE1.column1, count(ATABLE2.column1) 
    from ATABLE1 Left OUTER JOIN ATABLE2
         on ATABLE1.column1 = atable2.column1 AND atable2.column2 = '1'
    GROUP BY ATABLE1.column1;