No bananas, no oatmeal. Try applesauce, prunes (I know you said you've tried, but give it some time and keep trying them) and apricots, pears, plums and peaches also are great. Add more fiber foods . Fiber foods for older babies are bran cereals, graham crackers, whole-grain breads and crackers, and high-fiber vegetables such as peas, broccoli, and beans.
I also found this on askdrsears.com
"CONSTIPATION IN INFANTS UNDER A YEAR
In addition to the above general tips for preventing and treating constipation at all ages, try these infant-specific tips:
If formula-feeding, experiment with different formulas to find which one is most intestinal-friendly.
Feed baby smaller amounts of formula more frequently, which gives the intestines a better chance to properly digest the formula. An easier rule of thumb is to feed half as much twice as often.
Delay introduction of solid foods, especially more constipating ones, such as rice and bananas. Instead of rice cereal, try barley cereal. Good starter high-fiber foods for constipated infants are pureed pears and prunes.
Ease the passage of stools using glycerin suppositories or liquid glycerin (as described above).
Add one teaspoon of flax oil once a day mixed into baby cereal or baby's bottle.
Watch for about-to-go signs. As soon as your baby begins to grunt, grimace, look bloated, or show signs of straining, quickly insert a glycerin suppository to ease the passage."