Revenue and taxation; income tax; taxable income; business entities; computation; effective date.

HB 3603 modifies Oklahoma's income tax calculation rules for corporations and individuals. It adjusts taxable income to account for interest on state/local obligations, deducts amounts state cannot tax due to constitutional or federal restrictions, and revises how businesses carry forward tax losses (net operating losses). The bill also details how income from property (like rents or investments) and business activities should be allocated based on location, with specific rules for out-of-state sales and partnerships. These changes directly affect businesses operating across state lines and taxpayers with income from multiple jurisdictions. The bill aims to align Oklahoma's tax calculations more closely with federal rules while maintaining state-specific adjustments.