He wrote his MA thesis (1995-1996) and PhD dissertation (1998-2003) on medieval Spanish history under the supervision of Benjamin Ze’ev Kedar.
In his book La Era Mozárabe, Olstein asserts that after a century of self-imposed segregation, by the 1180s a process of intermingling between these two societies started evolving, reflected in the gradual demographic homogenization of the landscape, the growth of economic and neighboring relationships between communities, and the increasing rate of inter-community marriages.
However, amidst its own assimilation, the Mozarab community was able to acculturate the northern Christians by providing them with part of their Arab and Muslim economic, legal, and notarial legacies.
[5] Subsequently, his historiographical interests gravitated towards the varieties of macro-historical approaches - such as the world-system, historical sociology, and world history - that study the past on larger scales of space and time.
Beyond their singularities, the book arranges these twelve branches under the four big C’s for thinking history globally: comparisons, connections, conceptualizations, and contextualizations.
Volume 5: Expanding Webs of Exchange and Conquest, 500-1500 CE), Olstein mapped the connections throughout the Eastern and Western hemispheres concluding that the Middle Millennium (a more ecumenical concept than the European Middle Ages for referring to the period 500-1500 CE) was made of a multiplicity of tiny local worlds, in which, nevertheless, regional and even hemispheric forces such as conquest, trade, and religious conversion had had defining impacts on local societies.