Situations where RBO performs better than CBO are generally ones where every access is done by index. If CBO fails to perform in these circumstances it may be that it is rating the cost of index access too highly (of course, out of date segment statistics may also be an issue).

The init parameter that most strongly determines how aggressive the CBO is in choosing index access over table access is optimizer_index_cost_adj. The default value of 100 is way too high -- better results generally occur with values down around 20, or even less.