Yes, you can buy an O2 SIM and use it in your stick. But I would recommed you buying a Lidl SIM, through which you'll get exactly the same monthly flatrate (with speed throttling after exceeding 5GB) for just € 14.99/month. Besides the monthly flatrate you can also opt in for their dayflat, which gives you unlimited internet access for € 1.99/day.
Btw I have a spare Lidl SIM here, which I bought for the promotional price of € 5 and which I offer profitlessly: http://www.prepaidgsm.net/forum/buy-...activated.html
There's no provider in Germany offering completely unlimited mobile internet access - all throttle speed at some point, which usually is 5GB.