Completely breastfed babies do not get constipated. It might be the oatmeal. By the way, baby cereal is not nutritious, so unless she just loves it there is no real reason to feed it to a breastfed baby. It very well may be what is causing the problem.
You don't want to rely on things like prunes or prune juice to help your baby go. It's not necessary and may make your daughter dependent on prunes. It's completely normal for a breastfed baby to go 10 days without a bowel movement! The problem is if the poo is very hard when it comes out, not if it takes a week or two. Frequency of poo doesn't equal constipation, but if it's hard or she seems ill, then she very well might be.
There is enough water in your breastmilk that she doesn't need extra, so don't worry if she's not taking water or juice. Juice isn't very nutritious so there's no need to feed it to her.
Here is a quote from an IBCLC on constipation and the breastfed baby:
"Constipation occurs when the stool is hard and dry when it is passed, not when baby has infrequent stools. A constipated baby might be ill, especially if he is also vomiting frequently, does not want to breastfeed, seems weak and has a dry mouth. If your baby has these symptoms, get immediate medical care.
Babies and toddlers can get constipated from cow’s milk based formulas, cow’s milk and rice cereal. Exclusively breastfed babies rarely get constipated, but once other foods are added the stools get firmer. The baby’s doctor, a registered dietician, or an IBCLC can help you figure out which foods might be causing a problem. Check with the baby’s doctor if you are concerned about your baby’s pattern of bowel movements."
Generally, if a breastfed baby is truly constipated, it is from other solids OR dairy in your diet. Did she have issues before the cereal? If not, then it is not your diet. If she did, though, you might want to cut dairy out to see if that helps.
Again, don't rely on things like apple and prune juice, nor prunes, to help her go. And you don't want to do things like putting a thermometer in her anus to "stimulate" things - people recommend that a lot, but it is NOT what you should do. And I've heard of giving Karo syrup - NO! Why on earth would you give that to a baby?!? Again, it is relying on artificial stimulants when she probably doesn't need that, and could make her body reliant on those things in the future. Karo syrup causes diarrhea, it shouldn't "cure" constipation in a little one.
If your doctor is pro-breastfeeding (many are not) then talk to her/him about what baby is eating that might be making her constipated. If not, then talk to a La Leche League leader or a lactation consultant.
And again, if you're concerned about the frequency, there is no need to be. If the poo is hard (it WILL get firmer with solid foods, but not hard) then you are looking at constipation.
Good luck!