If using wooden skewers, soak them in water until ready to use.
In a large bowl, gently mix together the marinade ingredients: olive oil, balsamic vinegar, salt, pepper, garlic powder, and oregano.
Wipe the mushrooms clean with a damp paper towel. If they have prominent stems, remove them.
Cut the bell pepper and red onion into squares.
Slice the zucchini into ¼-inch thick slices.
Add the prepared vegetables to the marinade and gently mix until well-coated.
Heat your grill on medium-high. I use an indoor dual-contact grill. You can use a similar grill, an outdoor grill, or a grill pan.
Thread the vegetables on six skewers.
Oil your grill and place the skewers on it. Grill the vegetables until tender, 4-5 minutes per side, or 3 minutes per side if using a dual-contact grill.
Serve immediately. They are good hot or at room temperature.
