KYIV—Ukraine’s parliament passed a law on Thursday to order “oligarchs” to register and stay out of politics, a day after an attempt to kill a top aide to President Volodymyr Zelenskiy, which officials said could have been a response to the reform. The law provides a definition for an oligarch and gives a body headed by the president, the National Security and Defense Council, the power to determine who meets the criteria. Oligarchs would be forbidden from financing political parties or taking part in privatizations. Top officials, including the president, prime minister and head of the central bank, would be required to declare dealings they had with them. Zelenskiy says it is necessary to protect the country from powerful businessmen who have corrupted its political system for decades. His opponents say they fear it will be applied selectively to concentrate more power in the president’s hands. “Thanks to the anti-oligarch …