Whenever an OKR/Project needs an update and you want to leave a comment specifically as a reminder to check in or update the OKR, you can now click on the progress icon on the left of the progress bar on the OKR/Project list, and nudge the person who is responsible for making the check-ins.
The nudge option is available in the progress popover when you click on it. It will be visible to the users who are not the primary check-in owner of the OKR.
In the popup, you can also see how the notifications for that particular user are configured and where they will be notified. The nudge will go out on Slack if integrated for their account, otherwise, it will fall back to email.
For Slack users, the nudge is received by the Ally Slackbot and looks something like this: