Research Highlights: Thermophilous oak forests are among the most species-rich forest ecosystems in Central Europe. In the temperate zone, they evolved from mixed deciduous forests due to centuries-long livestock grazing. The abandonment of traditional forms of landscape use resulted in a constant decline in the number of patches of these communities, their area and species richness, which has been ongoing for decades and calls for their urgent conservation. The commonly used approaches to the conservation of this community are the reestablishment of grazing or mechanical removal of undergrowth. However, there are a limited number of works comparing their effects on the forest herb layer separately and in combination. Background and Objectives: The purpose of our research was to evaluate the effectiveness of grazing, mechanical brush removal and their combination for the conservation of the oak forest herb layer. Materials and Methods: Our work was based on a fully crossed experimental design set in a 60-year-old oak forest. The individual and combined influences of sheep grazing and brush cutting on forest floor vegetation were compared to control plots. We surveyed plant species twice—before the application of treatments and one year later on 600 one-square-meter subplots selected randomly in the limits of twelve fenced 20 m × 20 m treated and untreated study plots. Results: Both grazing by sheep and mechanical removal served well for total plant species richness and their cover, if applied separately. But these effects were not additive—plant species richness and plant cover on plots with combined treatment did not differ from plots, where just a single treatment was applied. Application of both treatments (but separately) had positive influence on species cover of the target group of plants typical to xerothermic oak forests and non-target species of mixed deciduous forests. Mechanical removal allowed also for successful control of woody species. Active conservation measures resulted also in negative effects—we observed increase in the species richness and cover of ruderal species on grazed plots. Conclusions: Both tested methods can be used for active conservation of open oak forest understorey vegetation. The method of active conservation should be chosen depending on the goal and the species composition of the forest floor and undergrowth found at the beginning of the restoration process, however, combining of these treatments does not bring any extra advantage. In our opinion a monitoring of the reaction of vegetation on treatments is of paramount importance.