Both these tests vary in terms of difficulty. However, some consider OET an easier test. This is because it is specific to healthcare and the test-takers can make use of their healthcare knowledge, vocabulary, and experiences. IELTS, on the other hand, is not specific to the healthcare profession. So, for a healthcare professional, OET may be a bit easier, Nevertheless, OET is still an English ​language proficiency test and not a test of healthcare knowledge.. 
Most recognising boards and councils require candidates to score at least 350 in each of the four sub-tests - Listening, Reading, Writing and Speaking. A score of 350 is grade B. Nevertheless, it is better to check with the concerned board or council regarding the minimum required score. For instance, for nurses, some boards require a minimum of C+ in the Writing  section. However, a minimum of grade B is required in other sections. For doctors, a minimum of grade B is required on all four subtests. 
Just like IELTS, OET certificates are valid for two years.
The OET certificate must be valid at the time of submitting your application to the concerned body in the country you wish to work in. 
The OET test can be attempted as many times as the candidate wants to improve his score. The OET test is held twice in a given month. What documents do candidates need to carry to the exam centre on the day of the exam?