We examined the effect of galactoglucomannan oligosaccharides (GGMOs) and/or Cd on the activity of peroxidases and the proteome of maize (Zea mays L.) roots and leaves. Our previous work confirmed that the application of GGMOs ameliorates symptoms of Cd stress in plants. Here, the plants were cultivated in hydroponics for 7 days, and the protein levels and peroxidase activity were estimated in intracellular, basic cell wall, and acidic cell wall protein fractions. The activity of peroxidases varied between the plant organs, as well as among the fractions and treatments. The application of GGMOs on plants suffering from Cd stress did not significantly influence the levels of peroxidases but modulated their activity, which implies posttranslational regulation. Next, we focused on the characterisation of proteins with significant changes in their levels in the GGMOs + Cd treatment. The changes in the level of various proteins (e.g., related to the defence reactions, cell wall structure/metabolism, and activation of plant hormones) caused by GGMOs and Cd presence suggested possible mechanisms of action of these oligosaccharides, which improve vitality of maize seedlings exposed to Cd.