It was discovered on 28 September 1935, by Dutch astronomer Hendrik van Gent at Leiden Southern Station, annex to the Johannesburg Observatory in South Africa.

[9] The stony S-type asteroid belongs to the Koronis family, a group consisting of few hundred known bodies with nearly ecliptical orbits.

[9] American astronomer Richard P. Binzel obtained a rotational light-curve of Onnie from photometric observations in September 1983.

In 2011 and 2013, respectively, a modeled light-curve using data from the Uppsala Asteroid Photometric Catalogue and other sources gave a period 23.0447 hours, as well as a spin axis of (183.0°, -75.0°) in ecliptic coordinates (U=n.a.).

[7][10] According to the 2014-published result by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Onnie measures 13.77 kilometers in diameter, and its surface has an albedo of 0.198.