Hi fario,
I am glad to hear you and your girlfriend are being really thoughtful around pregnancy risks and safer sex practices!
If you are and your girlfriend have been each other's only sexual partners (including oral, vaginal, and anal sex) than it is highly unlikely that either of you have an STI.
But, there are a few caveats to that. STIs are called "sexually transmitted" infections because that is how they are most often transmitted, but some STIs can be transmitted non-sexually.
- Oral herpes is most commonly transmitted non-sexually, usually in childhood.
- HIV can be transmitted through breast milk if the mother is HIV+, and can also be transmitted through IV drug use or use of a dirty needle in tattooing or piercing.
- Hepatitis can also be transmitted through IV drug use and use of a dirty needle. Hepatitis A can be transmitted through ingestion of contaminated food/drink, and Hepatitis B can be transmitted through birth to an infected mother.
This information is, primarily, from this advice column:
http://www.scarleteen.com/article/advic ... ave_an_std
So if you would like to be certain that neither of you have an STI getting a full round of testing at a sexual healthcare provider is your best bet. You have both been sexually exclusive for more than six months (many more!), and it sounds like are planning to continue to be monogamous, so if neither of you tests positive for any STIs you could stop using condoms without any concern for STI transmission.
If you decide to stop using condoms when she gives you oral sex, you two will want to talk about what you both are comfortable with in terms of ejaculation--for example is she comfortable having semen in her mouth, is she comfortable having semen on her body etc.
Does all that make sense?