You can control the distribution of resources between OUs and projects by allocating funds. Funds are allocated to OUs and then divided between projects using project spend plans.
Keep in mind that funds cannot be moved from one "sibling" OU to another at the same level - they must be moved from parent to child. You can move funds from a child back to a parent OU using the steps outlined in the article titled Recall Funds from an OU. For more information on fund allocation, see the Allocate Funds to an OUarticle.