I get mixed feedback from Polish people on this. Most people claim 'i' and 'oraz' are completely interchangeable but then suggest some subtle additions to that rule!
"i" is the default for "and". Generally the first time you use "and" in a statement you use "i". However if you need to use "and" a lot you might break up the monotony by using "oraz" from time to time.
"Oraz" is more common in the written form of the language but you do hear it spoken from time to time.
"Oraz" can also be used to mean "as well as" in a sentence like "Robert and Magda as well as Mark and Maria" - "Robert i Magda oraz Mark i Maria". Here we see it used to create two separate groups of people.