John Anthony Olszewski Jr. (/oʊˈʃɛski/ oh-SHESK-ee; born September 10, 1982), also known by his nickname Johnny O, is an American politician who has served as a member of the U.S. House of Representatives from Maryland's 2nd congressional district since 2025. A member of the Democratic Party, he served as the 14th county executive of Baltimore County, Maryland, from 2018 to 2025.

Olszewski served as a student member of the Baltimore County Board of Education from 1999 to 2000, and two terms in the Maryland House of Delegates representing the 6th district from 2006 to 2015. The district covered most of the southeastern portion of Baltimore County, including the town of Dundalk, Maryland. Olszewski unsuccessfully ran for the Maryland Senate in District 6 in 2014, narrowly losing to Republican challenger Johnny Ray Salling. Olszewski became Baltimore County Executive in 2018 after narrowly defeating state senator James Brochin in the 2018 Democratic primary election and defeating former state Insurance Commissioner Alfred W. Redmer Jr. in the general election. Olszewski was re-elected in 2022, defeating Republican nominee Pat McDonough.

Olszewski was elected to the U.S. House of Representatives in 2024, after defeating Republican radio host Kimberly Klacik in the general election. He was sworn in on January 3, 2025.

Olszewski was the eldest of three sons born to father John Olszewski, a lobbyist and a former member of the Baltimore County Council from 1998 to 2014, and mother Sherry Olszewski (née Crusse). He was raised in Dundalk, Maryland, and graduated from Sparrows Point High School in 2000.

Following high school, Olszewski attended Goucher College, from which he graduated with a bachelor's in political science and American studies. After college, Olszewski pursued a master's in political management from George Washington University, which he obtained in 2006. In 2017, Olszewski earned a doctorate from the University of Maryland, Baltimore County.

Olszewski was appointed to the Baltimore County Board of Education as the student member of the board, serving from 1999 to 2000. He was later elected to a term on the Maryland Democratic State Central Committee, serving from 2002 to 2006. After graduating from Goucher, Olszewski began teaching social studies and special education at Patapsco High School and Center for the Arts for five years and worked as a countywide resource teacher for two years.

In June 2006, Olszewski was nominated by the Baltimore County Democratic Central Committee to fill the seat left by the resignation of John S. Arnick, who had been appointed to the Maryland Board of Contract Appeals. Governor Bob Ehrlich appointed Olszewski to the seat on June 10, and he was sworn in on June 12. Olszewski was elected to a full four-year term in 2006 and subsequently re-elected in 2010.

In the legislature, Olszewski was a member of the Environmental Matters Committee in 2006, afterwards serving in the Ways and Means Committee until 2010 and then the Economic Matters Committee until 2015. In 2011, he was elected as chairman of the Baltimore County Delegation, making him the youngest person to ever fill that role. In 2012, Olszewski served as a delegate to the Democratic National Convention, pledged to President Barack Obama.