A 34-year-old man from Liverpool is being questioned by Merseyside police after he was arrested on suspicion of murder and attempted murder in the case.
A man aged 41 from the Knowsley area of Merseyside was arrested on suspicion of assisting an offender and is also in custody.
The men were held in the Runcorn area in the early hours of Sunday, police said.
Olivia died after she was shot in her home when a gunman chased convicted burglar Joseph Nee into the property at about 10pm in Dovecot, Liverpool, on August 22.
She had been at home with her two older siblings and mother, who opened the door after hearing gunshots outside. Her mother Cheryl was also injured.
A postmortem examination found the cause of death was a gunshot wound to the chest.
“I continue to urge anyone who has information that can help our investigation into Olivia's tragic murder to please come forward so we can bring those responsible to justice,” said Detective Chief Superintendent Mark Kameen.
“A number of people have been arrested in respect of this investigation.
“However, we still need the public's help in ensuring that we can a build a strong evidential picture so justice is served for Olivia and her family.
“With this, I appeal for those who may have information, no matter how small, that could continue to help our inquiries to contact us directly or anonymously and we will do the rest.”
A man aged 36 from Huyton and a 33-year-old from Dovecot who were arrested on suspicion of murder and two counts of attempted murder in relation to Olivia's death have been released on bail.
The force also appealed to the “criminal fraternity” in Liverpool for information, saying that the incident “crosses every single boundary”.
Police made more than 30 arrests in 24 hours a week ago as part of a crackdown on organised crime after the girl’s death.