I received os from men. used pep for 28 days. test hiv 4th generation at 27 days after pep with negative. is this conclusive?

Presumably OS means oral sex? If so, it was very low risk for HIV: there have been very few if any proved cases of HIV transmitted by cunnilingus (oral-vaginal contact); it's surprising you were prescribed PEP. However, for conclusive testing you'll have to wait a while. If PEP doesn't work, it can delay positive test results to 6 weeks or maybe even 3 months after the last PEP dose.