In recent years, new plant-based foods and drinks have been developed to meet the growing demand for animal-derived alternatives, particularly dairy products. This study investigates the impact of lactic acid fermentation on the organic acids and sugars in oat and buckwheat beverages developed using Lactobacillus johnsonii K4 and Lacticaseibacillus rhamnosus K3, which are potential probiotics. The fermented samples were analyzed for pH changes, bacterial viability, and the concentration of organic acids and sugars over 15 days. The results indicated significant variations in bacterial colony counts, with L. johnsonii K4 showing the highest initial growth. Over 15 days, pH levels decreased, with the most acidic conditions observed in buckwheat beverages. Notably, fermentation led to a significant increase in acetic acid concentration and a reduction in malic acid levels, particularly in buckwheat samples. These findings highlight the dynamic nature of fermentation in enhancing the nutritional profile and shelf-life of plant-based beverages.