Couple of points to add to previous responses:
If you haven't done basic highschool maths in a while, I would revisit the topics of geometry/trigonometry, probability and basic statistics.
While I agree GMAT quantitative section is a good way to practice, don't go overboard with buying GMAT material. Also, only do the "problem solving" questions - the OW test doesn't ask any of the "data sufficiency" type questions. You can find plenty of free content online - also, you can download the "Gmat test simulator" for free and do the math section. Unless you are planning on doing the GMAT soon as well, I would not invest into materials to prep for this.
Finally, being able to fast mental maths is not important, since you can use a calculator. So don't focus on this for the test (but definitely practice it for the actual interview).