Arctic Oscillation (AO) changes are inferred from a tree-ring reconstruction of a warm-season temperature index. The reconstruction covers AD 1650--1975 and is based largely on chronologies from circumpolar-Arctic and circum-North Atlantic areas. It accounts for 48% of the variance in the instrumental AO record from 1900 to 1975, verifies using independent data, and exhibits its largest variance at low frequencies. Positive levels during 20th century periods equal or exceed values back to AD 1650. Trends (including lower values during 'Little Ice Age' periods) resemble those of Arctic temperature reconstructions, reflecting some data overlap, but also the strong link between the AO and northern temperatures. A reconstruction of an AO summer sea level pressure index shows similar trends. Comparison of these reconstructions with proxies of the North Atlantic Oscillation (NAO) and other indices can help clarify relationships between the AO and NAO, at least during the boreal warm season.