For everyone else, it is by appointment only.
The New York Times reports:
On Mr. Kelly’s first day on the job, he held a small meeting with top aides to the president after a fuller staff meeting. He told them that Oval Office access to Mr. Trump, which was once nearly universal to people coming through the West Wing, would be strictly limited to appointments only.-RW
The exceptions, Mr. Kelly said, were the president’s wife and his 11-year-old son. And, he added, turning to Ivanka Trump, who was seated near him, the president’s eldest daughter, if she was speaking to him as a daughter and not a member of his staff.