Stocks making the biggest moves midday: Oracle, Nvidia, Immunomedics, Overstock & more

Check out the companies making headlines midday Monday:

Immunomedics — Immunomedics stock more than doubled on news that Gilead Sciences would buy the cancer drugmaker for $21 billion. The transaction is expected to close in the fourth quarter. Gilead will pay $15 billion in cash for Immunomedics, with the remaining $6 billion coming from newly issued debt.

Oracle — Shares of the cloud company popped more than 4% on news that TikTok-parent ByteDance picked Oracle to be the U.S. partner of the popular social video app. The deal is awaiting government approval. "Oracle confirms [Treasury] Secretary [Steven] Mnuchin's statement that it is part of the proposal submitted by ByteDance to the Treasury Department over the weekend in which Oracle will serve as the trusted technology provider," Oracle said Monday.

Nvidia — Nvidia surged more than 6% after the company said it will buy fellow chipmaker Arm Holdings from Softbank. The deal will include $21.5 billion in Nvidia stock and $12 billion in cash, including $2 billion payable at signing.

Delta Air Lines — The airline stock rose 2% after the company announced a deal to raise $6.5 billion of debt backed by its frequent flyer program. The move comes as negotiations in Washington for another relief package, which could include more money for airlines, remain gridlocked. United Airlines announced a similar plan in June.

Overstock.com — Shares of Overstock.com surged nearly 12% after Needham initiated coverage on the online retailer with a buy rating. The Wall Street firm said Overstock "is benefiting from external and internal factors that are driving an acceleration in revenue growth and market share gains." The ongoing secular shift towards digital commerce will continue to accelerate, Needham added.

Kroger — Shares of Kroger fell 4% after Bank of America downgraded the grocery chain to neutral from buy. The bank said Kroger's valuation is "not likely to expand" and that its momentum from the pandemic boost is "slowing."

Nikola — Nikola shares were up more than 6% in volatile trading as the electric truck maker continues to battle fraud claims from shortseller Hindenburg Research. In a statement, the company said Hindenburg's accusations are "designed to manipulate the market to profit from a manufactured decline in Nikola's stock price."

Micron Technology — Micron shares jumped more than 7% after a Goldman Sachs analyst upgraded the chip maker to buy from neutral. The analyst said Micron is "well-positioned competitively as it continues to execute on its tech transitions."

Thermo Fisher — The biotech stock rose 1.6% after Morgan Stanley initiated coverage of the company with an overweight rating. The firm said in a note to clients that it expects Thermo Fisher to "meaningfully outpace peers." It set a $485 per share price target, 12.6% above where shares closed on Friday.

Pfizer — Pfizer shares traded more than 2% higher after CEO Albert Bourla said the company could deliver its coronavirus vaccine to Americans by year-end. He also said they should have key data on the drug from its late-stage DBA trial by the end of October.

America Movil — The U.S.-listed shares of the telecom company rose nearly 6% on news Verizon will acquire Tracfone — the largest reseller of wireless services in the U.S. — for more than $6 billion. Verizon expects the deal to close in the second half of 2021.

—CNBC's Yun Li, Maggie Fitzgerald and Jesse Pound contributed to this report.