The objective was to evaluate the use of 2 types of experimental silos to characterize microbial counts, fermentation, and aerobic stability of oats ensiled with and without addition of a homolactic and heterolactic inoculant. From each of 6 sections in a field, whole-plant oats at heading stage were harvested, wilted, chopped, treated (INO) or not (CON) with inoculant, packed into 20L plastic bucket silos (BKT) or 15.2 × 30.5cm nylon-polyethylene embossed vacuum bags (BAG), and ensiled for 217d. The inoculant added contained Lactobacillus buchneri and Pediococcus pentosaceus (4 × 10⁵ and 1 × 10⁵ cfu/g of fresh oats, respectively). Experimental design was a complete randomized design replicated 6 times. Treatment design was the factorial combination of 2 silo types × 2 inoculation treatments. At d 0 there were no differences between BAG and BKT, and between INO and CON on DM (44.0 ± 1.0, %), pH (6.11 ± 0.07), and counts of lactic acid bacteria (9.28 ± 0.13), yeasts (4.88 ± 0.09), and molds (3.97 ± 0.08). At opening (d 217) there was no difference in DM (42.9 ± 0.9, %) among treatment combinations. There was a lower pH (4.25 vs. 4.41 ± 0.02), yeasts (1.10 vs. 4.13 ± 0.34), and NDF (65.0 vs. 67.0 ± 0.40) for INO compared with CON, respectively (P ≤ 0.05). Lactic acid bacteria count (6.39 vs. 5.65 ± 0.17), DM recovery (96.1 vs. 92.9 ± 0.43, %), and aerobic stability (565 vs. 133 ± 29, h) were greater in INO compared with CON, respectively (P ≤ 0.05). Molds were similar when INO was applied to BKT (1.68) and BAG (1.42) but higher in BKT (2.82) compared with BAG (0.80 ± 0.41) for CON (silo type × inoculation, P ≤ 0.05). In summary, differences due to silo type were only significant for mold count at opening. The combo inoculant used increased lactic acid bacteria, decreased pH and NDF, and improved aerobic stability by decreasing yeast count of oats silage. Silo bags are an alternative technique to buckets to adequately characterize lactic acid bacteria, yeasts, and fermentation of ensiled oats.