Treating the surface of the ears with peroxide will only make things worse. The gooey putrid black fluid is probably coagulated blood heavily contaminated with bacteria and yeast. The ears will first need a cleaning by the Vet, before Ottomax can have any effect. In addition since the infection is severe you'll need Antibiotics such as Cephalexin to treat from the inside out. Probably 3-5 days you'll see dramatic improvement.
Then you can address the underlying cause. A Lab's ear has the length and width that enhances the entrapment of moisture. Perfect environment for bacteria and yeast. My previous dog a 1/2 Lab, 1/2 Golden would frequently pick up bacterial/yeast infections during the summer from the pond. The equivalent of swimmer's ear. A good preventative was to treat with a 1:3 White Vinegar to water solution, lowering the pH making for a less favorable environment for bacteria and especially yeast. There are also commercial cleaners you can obtain from your Vet. Likewise the dog may have an environmental allergy that is causing the initial scratching. In this case giving Benedryl when the itching begins along with Ottomax can prevent additional problems. As another poster points out, ear mites can be an underlying cause.
Once the ecology of the dog's ear is disturbed by infection, it will be prone to infections there after.
Your plan should be to work with your Vet to get rid of the apparent severe infection, and then look for the underlying cause to minimize reoccurance.