What is the longest downrod available for the Oslo fan?.

72 inches is the longest downrod manufactured by Oxygen Lighting. You'll need to go to this page, then hover over each one to find the right finish color you require, lastly there is a dropdown menu on the page that will allow you to select the correct length. https://www.oxygen.lighting/collections/ceiling-fan-downrod

Does the Oslo hugger fan install on sloped ceilings? If so, does it need a sloped ceiling adapter?

All Oxygen ceiling fans, with the exception of the hugger models (obviously), are designed to install on sloped ceilings with a slope of up to 20 degrees. Oxygen doesn't make a sloped ceiling adapter for this reason.