The ratings are down significantly since Glor left. Down to 5.2 million viewers from about 6 million when Glor was helming it.
"ABC’s “World News Tonight” topped the evening newscasts with an average of 8.2 million viewers. NBC’s “Nightly News” was second with 7.4 million and the “CBS Evening News” had 5.2 million viewers."
https://www.apnews.com/c2348b715a594727b8b27435f30c4360
In 2018 there was hand wringing over a drop in Jeff Glor's ratings which were significantly higher than what O'Donnell is bringing in:
https://pagesix.com/2018/05/31/cbs-may-boot-jeff-glor-from-evening-news/
Despite this worrisome performance, CBS might have to stick with O'Donnell. I can't think of who might come off the bench and there would be negative publicity from letting the only female network newscast primary anchor go. The only thing that might have a chance at reversing this would be a misstep by an anchor at either ABC or NBC.