Legislative power in the State lies with the State House of Assembly (SHA) composed of elected Constituency Members and assisted by Support Staff. The Oyo State House of Assembly has members who are elected for tenures of 4 years to represent the various State constituencies.
The State House of Assembly has;
- The Speaker
- The House Leader
- The Chief Whip and;
- The House Committee Chairman as key officers.
The Legislature has oversight function over the Executive arm, and the Local Government Councils (LGCs) of the State.
FUNCTIONS OF THE STATE HOUSE OF ASSEMBLY
 Law Making
Principally, the State House of Assembly is to make laws for the peace, order and good governance of the State in respect of matters not in the Exclusive legislative list but in Concurrent list. It could also legislate on other matters with which it is empowered by the Constitution to do so.
 Representative Function
Membership of the State House of Assembly is based on constituency arrangement of the State for effective representation of the populace. Each member therefore represents a section of the people in the State (Constituents) for avoidance of marginalization in the scheme of things.
 Deliberative Function
Before any Bill is passed into law, or a resolution taken on any matter before the House, members must procedurally, engage in a careful and meticulous deliberation of all the facts therein, to arrive at definite and worthy decisions.
 Administrative Function (Confirmation of Appointments)
The House of Assembly participates in the appointment process. Under Section 192(2) persons nominated for appointment as Commissioners, State Chief Judge, Board Members and Chairmen of Parastatals etc. require the Confirmation of the State House of Assembly for such appointments to be valid.
 Oversight Investigative Function
This function is contained in Section 88 of the 1999 Constitution. Here the State House of Assembly is empowered to carry out investigations within its competence to prevent and expose corruption, inefficiency or waste in the execution or administration of laws. It is empowered to procure evidence, written or oral, direct or circumstantial as it may deem necessary and also to examine any witness whose evidence may be material to the subject matter. The primary aim of this function is to promote efficiency, transparency and due process in the running of the affairs of the State.
 Quasi-Judicial Function
The State House of Assembly performs this important function through the impeachment and removal from office, of members of the executive. Under section 143 and 188′ of the 1999 Constitution, the State Governor and his Deputy are liable to impeachment. This is the process by which the legislature formally brings charges against the Governor or his Deputy with a view to his trial and removal from Office if found guilty as charged.
 Judicial Function
The State House of Assembly is empowered to regulate its Proceedings including control over the conduct of members during Sessions. Such members or even non-members may be tried summarily for contempt and punished without the right of appeal to law Courts. Again the State Assembly can conduct a trial as if it were a Court of Law. It may thus issue a Warrant to compel the attendance of any person who, after having been summoned to attend, fails to do so.