Reeva was still alive...
when Oscar told Mr. Baba that "everything is fine."
Recall from the BBC3 doc on this matter, DT's own pathologsit Dr.Perumal stated that in his opinion after going to the crime scene that "Reeva died on the stairway."
But this was a few minutes after Oscar told Baba that 'everything is fine' as it took a few minutes for him to realize something was amiss and to drive over. Once there he could see Oscar carrying Reeva down the stairs.
Couple this with Perumal's statement. Perumal did not explain, but I presume he refers to the lack of blood after a certain point on the stairs, implying her heart was no longer beating. JMO.
So Reeva was still alive when Oscar said, "security, everything is fine." But dying alas.
To me the key thing here is that "Everything is fine" only truly becomes damning if it resulted in a material delay in initiating medical assistance. Did he call after carrying Reeva down the stairs when there were people there? Before on his own? What was the gap between his statement to Baba and the call? And on which side? I don't know.