Acorns contain tannins that cause GI upset and, in larger amounts, kidney injury.
Like conkers, acorns are also a foreign-body obstruction risk.
Acorns and oak leaves contain hydrolysable tannins (gallotannins) that irritate the GI mucosa. Larger ingestions can damage the renal tubules and liver. Acorn hulls are also a recognised cause of intestinal obstruction in dogs.
Surse
Small numbers of acorns usually cause only GI signs; large ingestions or repeated exposure can damage kidneys.