Oral sex is considered mutual masturbation when it is used as a substitute for sex.
However, this is the wrong way to think of the question. The problem is that the spirit of chastity is not how much I can get away with it without having sex, but how much am I willing to sacrifice in defence of my girlfriend’s honour and purity. Are we instruments of freeing each other to learn to look deeper into each other as a person, or rather are we encouraging each other to stop at the surface and mutual and selfish gratification? There is a reason why we should stay pure before marriage and outside of marriage, not because marriage is the license to sex, but because only when we can first love with purity, then we can truly give ourselves freely away to each other in marriage. And without that, we don’t yet know how to love well, and receive the fruit of sexual union with love. The sad reality is, it is because of this that many who are married either turned the marital union into gratification or obligation, or both; husbands and wives are not trained to give each to the other in freedom, but to take and to objectify each other through mutual usage.
There are many good books and programs out there today that can help you understand this better. One of the books I recommend you to pick up is by Christopher West “Good News about Sex and Marriage”, which you can read from start to end, or read the chapter on before marriage. It is very accessible, in Q&A format, and he provides a much more thorough explanation of the teaching than I can here.