Oral Sex ? Please help !

If two partners are clear from any STDs or STIs, is it still possible to get one from them or create one ? I asked a Pharmacist on the Walgreens app, & she told me that it’s not possible to receive one if there wasn’t an STD in the first place, I’m just looking for reassurance here. I’m talking about oral sex here. I don’t feel any different, I’m feeling my normal self. I just overthink on these types of things.. Thank you !