It seems that occurence is not a word, but a friend said it is because he says he found it in the OED.
You say that your friend found the word "occurence" in the OED (note that you weren't searching the OED in your link; the OED is a premium historical dictionary of English), which is technically true. It's listed as one of the forms of the word occurrence:
Forms: 15 accoranse, 15 accurauntes (plural), 15– occurrence, 16 (18– irreg[ular]) occurence
You will not find this spelling in "regular" dictionaries. It is considered to be a misspelling of "occurrence", as you can see in Wiktionary.
It seems occurrence used to be spelled with just one r.
This book is a reproduction of an important historical work.
The Occurence of Diseases of Adult Bees
On Amazon books, there are nearly 500 hits for occurence spelled with one r. Many of them are quite literally spelled the same way in the texts, and most are scientific textbooks from around 100 years ago.
On Google Books, I got 285,000 hits for the word. So, it is a word that is still in print or texts all over the internet. It may be obsolete, for many (or most) people, but it's still technically an English language word.
I happen to like old books quite a lot, so outdated spelling doesn't bother me at all.
Edit: the reproduction books are unreliable for use as sources. But there are enough instances of the word being spelled with one r, such as in all these scientific reports from NASA, just to name one: