Developing probiotic plant-based products with high probiotic count, functionality, and sensory appeal remains challenging. In this study, oat drinks pretreated with NaOH, phosphoric acid, α-amylase, sprouting, or their combinations were fermented using Saccharomyces boulardii alone, Lactiplantibacillus plantarum 299v alone, or a sequential process in which the live cells of S. boulardii-fermented oat drink were heat-inactivated (paraprobiotic) and subsequently fermented with L. plantarum. The results showed that the α-amylase pretreatment yielded the highest S. boulardii growth (7.04 Log CFU mL−1; p < 0.05). Moreover, in both single-strain and sequential fermentations, oat drinks pretreated with alkaline and alkaline-α-amylase combinations showed significantly lower L. plantarum counts than the other treatments (p < 0.05). Sequential fermentation of sprouted-α-amylase treated oat drink yielded the highest phenolic content (381.67 mg GAE L−1; p < 0.05). Both the oat drink fermented with L. plantarum alone and that produced by sequential fermentation achieved high probiotic counts (> 10 Log CFU mL−1) and favorable sensory scores (> 7), but sequentially fermented oat drink outperformed all other evaluated treatments, exhibiting balanced acidity, superior antioxidant activity, and negligible alcohol (< 0.5%). To our knowledge, this is the first report integrating paraprobiotic-probiotic concepts into oat drink, offering a novel, non-alcoholic strategy to enhance functionality, consumer appeal, and health-promoting qualities in plant-based foods.