I am at risk of getting hiv from oral sex, giving and receiving. after 84 days i went to get 3rd generation hiv lab test and got negative result with s/co index 0.485 . and negative for syphilis, hepatitis b, hepatitis c. am i safe?

There have been few if any scientifically documented cases of HIV transmission oral to penis or by cunnilingus (oral-vaginal contact), so you were not at risk. Your HIV blood test is conclusively negative, and your other tests are conclusive as well. You had a safe exposure and were not infected with any of the infections for which you were tested.

The risk of getting HIV from oral sex while small is not zero. If you wish to continue these activities, talk to your doctor about getting pre-exposure prophylaxis. Visit this site for information on this topic: https://www.cdc.gov/hiv/basics/prep.html