The Osmo Fire Base has access to the charging ports only for some models. The charging ports are typically on the top edge of the device, so in some cases the Osmo reflector will cover the charging port. Here is a summary by model.
Fire tablet models that allow charging while in an Osmo Fire Base:
Fire HD 10 (9th and 11th Generations)
Fire HD 8 (10th Generation)
Fire tablet models that don't allow charging while in an Osmo Fire Base:
Fire 7 (All Generations)
Fire HD 8 (8th Generation)