I have written before how I like to make my own salad dressings and sauce where possible, rather than buying a premade bottle of something. The same goes for marinades. It is not complicated so why buy something (even though there are many choices at the grocery store). It's so easy this is barely a recipe.
Place your boneless/skinless chicken breasts, enough for the size meal you're making, into a shallow bowl or baking dish. You can also use a Ziploc if you prefer.
Drizzle with a generous amount of olive oil and lemon juice, equal amounts of each. You can't really overdo it, just make sure the breasts are sitting in a pool of the liquid.
Sprinkle with salt, pepper, garlic powder and dried oregano. The amounts will vary depending on the amount of chicken you are using, so again, just eyeball it.
Turn the chicken over, and sprinkle the other sides with the salt, pepper, garlic powder, and dried oregano.
Chill in the fridge for at least 30 minutes or for a few hours. You'll want to turn the chicken every 30 minutes or so until you're ready to cook it.
You can grill this on your BBQ or cook in a frying pan. We like to add it to pasta (like this lemon pasta), salad bowl, or just eat it on its own. Last night we ate it with corn and potatoes.