The objective of this study is to determine the incompatibility between the FDRE Constitution and the Oromia national regional state constitution on the matter of adjudicating constitutionality issues and to identify the possible effects of this incompatibility. Hence the study tries to answer: what are the possible effects of these incompatibilities between the FDRE and Oromia state constitutions on the matter of constitutional adjudication? To what extent the organs established under the Oromia state constitution for the purpose of adjudicating and resolving regional constitutional disputes are competent? What are the impacts of excluding judiciary from determining the constitutionality of laws under the regional state constitution of Oromia? Is there any clear mechanism provided under the FDRE Constitution to reconcile the inconsistencies between the federal and regional laws? To answer these questions, the study analyzes related provisions of the FDRE Constitution and the state constitution of Oromia with constitutional adjudication; appropriately reviews the existing literature and critically analyzes practical cases. The findings of this study include: While federal and state laws must, at least, be consistent on basic democratic principles, like ensuring separation of powers, check and balances, independence of judiciary and rule of law in a federation, the state constitution of Oromia prohibits ordinary courts from reviewing the constitutionality of both laws of the parliament and subordinate enactments of the executive organs like regulations and directives contrary to the FDRE Constitution. The exclusion of courts from reviewing the constitutionality of subordinate legislations of the executive highly erodes the principle of constitutionalism, at regional state level of Oromia.