A US Embassy statement calls the December 12 ceremony "extra-constitutional" and urges the opposition to work within Kenya's laws to pursue electoral and other reforms.
The statement comes as the top US official for African affairs, Donald Yamamoto, visits the East African economic hub after it faced months of political turmoil, including a repeat presidential election. He met with government and opposition leaders.
The opposition calls its planned swearing-in of Odinga "lawful" and blames so-called "stolen" elections for polarizing the country.
The US statement also urges Kenya's security forces to show "maximum restraint" after months of deadly clashes with opposition supporters.