It takes approx 4-6 weeks to establish sufficient BB in an aquatic environment. A 2 week old tank with a large waste producing fish like an Oscar is gonna to generate ammonia quickly. I personally don’t know of any bottled BB that would convert high levels of ammonia & nitrite immediately to nitrate. Having 0 ammonia & nitrite & the presence of nitrate on your test strip though does indicate a completed cycle. The fin burn & disrupted slime coat must have developed prior to the addition of the BB.
I would recommend adding salt at 1tbsp/5 gal to stimulate slime coat regeneration, and maintain large weekly water changes to keep your nitrates <10. Replace the salt to the same concentration for 14-21 days then let it dilute out with your master changes. The fins and the slime coat should heal over time.