In order to give locked down assessments to students with iPads, they'll have to have the "Lockdown Browser" app installed before they begin the assessment in Otus.
If you're using a mobile device manager in your site or district, you can push this to all the devices using your MDM. Otherwise, you can manually download it to each iPad from the iOS App Store.
Once this is downloaded on the iPad, students can begin the assessment as normal by navigating to Otus and clicking through the assessment module to begin their assessment.
Once they see the assessment they are taking, they can click on it. A lockdown browser prompt should appear. Since the lockdown browser for iPad has already been downloaded, students should just click Take Assessment.
The first time they use the lockdown browser, students will be asked to give the browser access to the iPad's camera and microphone, as some assessment answers can require a video or audio recording. Have the students click OK to give those permissions.
Finally, the students will be asked to Confirm App Self-Lock. Have them click Yes, and the assessment should begin in the lockdown browser.
When they are finished, they will click submit at the end of the assessment, and they can click the home button on the iPad to return to their app view and re-enter Otus through Chrome or Safari.