Girlfriend had cervical cancer.can i still have oral sex?

Yes: I'm hoping this is a curable cancer or premalignant lesion ('intraepithelial neoplasia', 'carcinoma in situ', etc.) and that by the time you are reading this, it's been treated for probable cure. Oral sex is great for a relationship and the hpv virus that caused the cancer is no reason not to enjoy your time together. Most important is you both educate yourselves about exactly what's happening.