There is so much, it's hard to give just a few things. But I'll try.
1. Fevers are muy importante. Even if it's just a temp of 100.4, get ready for a FFWU (full fever workup).
2. Oncologic pain is different from every other kind of pain you've dealt with. High doses of strong drugs are going to become normal for many of your patients.
3. Familiarize yourself with safe sexual practices for patients on chemo. That's one area that most nurses (myself included) need more education on. A lot of times, patients won't ask docs those questions--they'll ask you. Be ready to answer them or at least find them some trustworthy info.
4. Oncology assessments are different from assessments you might have seen on med-surg floors. Some chemo drugs affect the nervous system and/or GI tract, so keep an eye on your preceptors and watch what kinds of questions they ask their patients and what freaks them out. That last part especially!
5. Watch your preceptors. They will be able to give you a wealth of information on how to succeed in this specialty. Listen to the stories of your patients. Some of them know their regimens better than their doctors.
6. Make sure you're keeping yourself healthy mentally and physically. Lots of onc patients are long-termers. You can get very attached very quickly. You'll need a mental cushion to fall back on, and physical exercise is often a nice outlet for stress and a good way to process feelings.
Best of luck!