Creating Indexes for Group By Fields?

Do you need to create an index for fields of group by fields in an Oracle database?

For example:

select * 
from some_table
where field_one is not null and field_two = ?
group by field_three, field_four, field_five

I was testing the indexes I created for the above and the only relevant index for this query is an index created for field_two. Other single-field or composite indexes created on any of the other fields will not be used for the above query. Does this sound correct?