Can cancer cause anything like bad breath or oral ulcer?

Yes: Although ulcers in the mouth can be due to trauma or "cold sores", oral lesions presenting as ulcers may be associated with oral cancer and must be evaluated sooner rather than later.

Oral cancer?: Can you be more specific of what type of cancer? Oral cancer can definitely show up as lesions in the mouth. Chemotherapy can also cause break down of oral mucosa.

Yes: Yes it can, depending on type of cancer, oral cancer can present itself as oral ulcer especially if you are currently or previously smoked or chewed tobacco, said that it is much more common to have oral ulcers and bad breath from aphthous stomatitis or oral herpes.

Possibly: When in doubt, get to your dentist asap. A biopsy or culture of any oral lesions is the best way to be preventative and help your peace of mind.