Will exercising a large block of NSOs push me into a higher tax bracket?

It can. The bargain element on NSOs is ordinary income stacked on top of your salary, so a big exercise in a single year can push the top slice of that income into a higher marginal bracket – 32%, 35%, or 37% – and may also trigger the 3.8% net investment income tax indirectly or phase out certain deductions. Because the U.S. system is progressive, only the income above each threshold is taxed at the higher rate, but a large lump can still meaningfully raise your blended rate. Spreading exercises across multiple tax years keeps more of the income in lower brackets, when your options' expiration and the stock outlook allow. Before a large exercise, project your full-year income to see exactly which brackets the spread lands in.