Residents of the Ukrainian town, occupied by Russian forces since February 2022, have reported a deepening food crisis.
Leo "expressed the hope that co-operation between both parties will allow for the delivery of food and medical aid, and for the voluntary and safe evacuation of the civilian population," the Vatican said in a statement about Monday's call.
Zelenskiy did not immediately release a statement about the conversation.
The Pope, who is recovering from lung cancer surgery, spoke on Friday with Russian President Vladimir Putin and asked him to arrange a ceasefire to facilitate the entry of aid into Oleshky.
The Kremlin said afterwards that Putin had agreed to co-operate with the Pope to organise supplies for the town but did not mention a ceasefire.
Leo, who became pontiff in May 2025, has met with Zelenskiy several times.
They last saw each other in December, at the Pope's residence in Castel Gandolfo in Italy, after which Leo urged European leaders to take a collective approach in negotiating peace between Russia and Ukraine.
Russian forces overnight launched one of thier broadest air assaults against Ukraine in months, firing 16 ballistic missiles at Kyiv and the surrounding region and more than 200 drones across the country.
Ukrainian regional governor Oleh Syniehubov said a Russian drone attack on Monday killed six people, including three children, in a park in a village in the northeastern Kharkiv region.
Syniehubov, writing on Telegram, had earlier said 15 people were injured in the strike southwest of the city of Kharkiv.
The drone hit a park where families were playing with children, Interior Minister Ivan Vyhivskyi said.
Vyhivskyi shared a blurred photo showing a child's body in a stroller.
Other pictures posted online showed emergency crews working around a damaged building.
