General Electric jumped 4 percent Monday after activist investor Nelson Pelts bought a stake in the company.
Twitter rose 5 percent after a long-running search for a full-time CEO concluded. Co-founder Jack Dorsey will have a second chance at leading the company.
The Dow Jones industrial average gained 188 points, or 1.1 percent, to 16,662 as of 11:45 a.m. Eastern time.
The Standard & Poor's 500 index rose 23 points, or 1.2 percent, to 1,975. The Nasdaq composite rose 45 points, or 1 percent, to 4,753.
Oil rose 2 percent to $46.70 a barrel in New York.
Bond prices fell. The yield on the 10-year Treasury note rose to 2.02 percent.