Oskar Fiala was a Czech painter, graphic artist, and creator of bookplates.

Oskar Fiala was born into the family of civil servant František Fiala and his wife Augusta Julie, née Weisseová. He was a student of Prof. Jakub Husník, who taught at a secondary school in Prague from 1876. According to Prokop Toman, he was a student of Julius Mařák. Information about his graduation from Julius Mařák's landscape painting course at the Prague Academy is not clearly confirmed. The almanac of the Academy of Fine Arts in Prague does not list his name among the graduates. He is also not listed among the 43 students of J. Mařák in the catalogue of the exhibition Julius Mařák and His Students (National Gallery in Prague, 1999). According to J. Ort (2018), he studied in J. Mařák's specialization program in 1897. He completed a study trip to France.

He taught drawing at various levels of the school system throughout his life. He taught at the municipal school in Líšnice in 1882–1883. He then taught at a continuation school in Zbraslav, where he moved in 1895. He also taught drawing at the Imperial and Royal Industrial School in Smíchov. From 1910, he was the headmaster of a boys' secondary school in Bubeneč. He was one of the founders of the Educational Union (1906-1925), where he served as the secretary of the art section. In 1911, his textbook Kreslení ve škole obecné (Drawing in Elementary School) was published. Later, in 1919 (posthumously), together with V. Drbohlav and J. Kysela, he co-authored a drawing textbook for elementary schools entitled Kreslení na škole národní (Drawing at National School).

He was one of the founding members of the Union of Fine Artists, with which he exhibited for many years (from 1898). He wrote professional articles for the magazine Dílo (e.g., “Dílu Mikuláše Aleše”). Initially (at least from 1894), however, he also exhibited with the Umělecká beseda (Artistic Society). He regularly participated in exhibitions of the Krasoumná jednota (Fine Arts Union) from 1894, and in 1908 he exhibited a large number of paintings at its jubilee exhibition in the Rudolfinum (Na Olšavě, Západ, Řečiště Sázavy u Kamenného Újezda, Kazín nad Berounkou, Mlýn v Brodku u Jílového). According to the Guide to the Picture Gallery in the House of Artists: Rudolfinum in Prague, in 1913 he had a watercolor painting, Valašský dvůr (Wallachian Farm), in the permanent exhibition.

On August 16, 1892, he married Anna Zelená (1870–??), born in Požarevac, Serbia, at St. Stephen's Church in Prague. The Fialas had a daughter, Milada (1895–??), who married Boullard and worked at the embassy in Paris.