The asteroid "2019 OK" was first identified as a possible
hazard only one day before it's close approach to Earth.
Because it came roughly from the direction of a full moon,
the glare made it difficult to spot.
The simulation below shows what might have been seen,
if we'd had a telescope in space at the right location.
The asteroid came out of the constellation Microscopium,
and a few of the constellation's stars are labeled.