Flemish painter of still-life and flower pieces. In 1596 he went to study with Andries van Baseroo and became a master in Antwerp in 1602. He married Marguerite Ykens on 8 January 1606 and carried on business as a cork merchant. He is specially noted for his paintings of oysters, which show a masterly feeling for colour and texture.
The esteem enjoyed by Beert is indicated by the large number of pupils he had, including, in 1610, Frans van der Borch; in 1615, Frans Ykens; in 1616, Paulus Pontius; and, in 1618, Jan Willemssen. Beert's son, Osias Beert the younger (1622–78), was also a painter and became a master in 1645.