With the emergence of COVID-19, many educational pillars have been altered from conventional ways to online solutions. The educational assessment has been administered in online environments despite all encountered challenges. This descriptive study aimed to uncover the online assessment challenges that were confronted. Furthermore, it intended to display the impact of these challenges on the assessment principles. A mixed-method approach was adopted for data collection. A survey was used to collect quantitative data from 60 academic staff at Sultan Qaboos University in Oman, and semi-structured interviews were conducted with four of them. The study found some challenges when applying online assessment such as learners’ refusal to turn on cameras, heavy teaching loads, cheating, the long time required for developing online assessment instruments, impersonation/dishonesty, assessing practical experiences, plagiarism, grades’ inflation, assessing group’s work, academic integrity and a large number of students per section. The study concluded that these challenges respectively threatened assessment principles of validity, efficiency, fairness, reliability and variability. Practitioner Notes 1. Reasons standing behind each of the found online assessment challenges should be explored and solutions for each particular challenge should be examined. 2. Principles of validity, reliability, fairness, variability and efficiency of the online assessment need to be addressed. 3. The study recommended applying various alternative assessment strategies embedded in the online course activities to reduce the likelihood of cheating and increase the validity and reliability of the overall assessment process. 4. Due to the found challenges of online assessment, academic staff should avoid conducting a single heavy-weighted strategy for summative assessment. 5. Teachers should attend to individual cases facing difficulties carefully in the online assessment contexts; and prepare flexible alternative assessment plans.