Order of Presidential Succession

Who becomes President if Obama dies, resigns, or is removed from office?

"While the Constitution, as amended by the 25th Amendment , provides that the Vice President will succeed to the nation’s highest office on the death, resignation, or removal from office of the President, it delegates authority for succession beyond the Vice President to Congress. Over the past two centuries, Congress has exercised its authority in three succession acts, in 1792, 1886, and 1947. It has also added to, revised, and clarified the succession process through Sections 3 and 4 of the 20th Amendment, proposed by Congress in 1932 and ratified by the states in 1933, and the 25th Amendment, proposed in 1965 and ratified in 1967. The Succession Act of 1947 , as amended, and the two constitutional amendments currently govern succession to the presidency."
Source: Congressional Research Service (CRS), "Presidential Succession: Perspectives, Contemporary Analysis, and 110th Congress Proposed Legislation" , assets.opencrs.com, Oct. 3, 2008

Office Office Holder (as of Oct. 30, 2015)
President Barack Obama
1. Vice President Joe Biden, Jr.
2. Speaker of the House of Representatives Paul Ryan
3. President Pro Tempore of the Senate Orrin Hatch
4. Secretary of State John Kerry
5. Secretary of the Treasury Jack Lew
6. Secretary of Defense Ashton Carter
7. Attorney General Loretta Lynch
8. Secretary of the Interior Sally Jewell
9. Secretary of Agriculture Thomas Vilsack
10. Secretary of Commerce Penny Pritzker
11. Secretary of Labor Thomas Perez
12. Secretary of Health and Human Services Sylvia Burwell
13. Secretary of Housing and Urban Development Julián Castro
14. Secretary of Transportation Anthony Foxx
15. Secretary of Energy Ernest Moniz
16. Secretary of Education Arne Duncan
17. Secretary of Veterans Affairs Robert McDonald
18. Secretary of Homeland Security Jeh Johnson