There is a great chance that Maria Margaretha Oosthuizen was the second wife of Jacobus Johannes du Preez:
Name: Jacobus Johannes Du Preez
Event Type: Marriage
Event Date: 29 Sep 1914
Event Place: Christiana, Bloemhof
Birth Year (Estimated): 1866
Spouse's Name: Maria Margaretha Oosttuizen
Spouse's Gender: Female
Spouse's Age: 32
Spouse's Birth Year (Estimated): 1882
Digital Folder Number: 007749185
Image Number: 02830
Citing this Record:
"South Africa, Transvaal, Civil Marriages, 1870-1930," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:QPHC-T3FD : 3 October 2018), Jacobus Johannes Du Preez and Maria Margaretha Oosttuizen, Christiana, Bloemhof; citing Marriage, , NARS "Civil registration (marriages - coloureds), Transvaal and Swaziland, 1898-1927." The National Archives and Records Service of South Africa, Pretoria.
Jacobus Johannes was a widower according to the original document.
This couple had a son named Johannes Jacobus born about 1917 in Warrenton, C. P. and died on 15 Jul 1960.
According to the death cert., his father was deceased and his mother was living in Warrenton, C. P. as can be seen on the original document.
Name: Johannes Jacobus Du Preez
Event Type: Death
Event Date: 15 Jul 1960
Event Place: Warmbad
Birth Year (Estimated): 1917
Father's Name: Jacobus Johannes Du Preez
Mother's Name: Maria Margareta Oosthuizen Du Preez
Spouse's Name: Johanna Christina Petronella Immelman Loedolff Du Preez
Additional Spouse's Name: Daphne Anna Catharina Dippenaar Du Preez Hunsinger
Other Name(s): CHILD_1:Maureen Antoinette Du Preez
Digital Folder Number: 004246898
Image Number: 00005
Citing this Record:
"South Africa, Orange Free State, Estate Files, 1951-2006," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FC3Q-85N : 6 August 2017), Jacobus Johannes Du Preez in entry for Johannes Jacobus Du Preez, 15 Jul 1960, Death; citing Warmbad, Master of the Free State High Court, Bloemfontein; FHL microfilm 004246898.
(Images of the original documents included in the FamilySearch citations)