publications - Can something published on arXiv or optimization-online.org be mentioned in my CV?

I was thinking about submitting one of my papers in arXiv or www.optimization-online.org and hence I was just wondering if mentioning the same in my CV would be appropriate. Is it done usually? If yes, how should I actually mention something like this in my CV? Kindly guide me (I have no previous experience in publishing papers and I'm an undergraduate student).