Off-load, which means to remove a load or burden, has quite a few antonyms. Instead of off-loading, you could opt to load up or add more weight to a particular object. Another option would be to carry or haul a load, rather than removing it. Embrace could be used as an antonym as well, meaning to accept or take on a load or responsibility. Similarly, undertake is another option, which means to commit to a task. To carry the load or bear the weight also works as an antonym to off-load. Ultimately, it depends on the context, but these antonyms can be used to expand the meaning of the original word.