1. Get yourself ready to listen. Think about what you might expect to hear.
2. Try to pick out key words - a speaker may stress these words or pause a little before or after the word.
3. Listen for signposting words - things like 'Firstly, I'll.....'
Try not to only focus on medical English listening texts. It's great to listen to texts about diseases and treatments but remember that you need the 'building blocks' of the conversation as well. For example, how do we vary what we say? We use modifying words - usually, sometimes, always, on the whole, generally speaking, now and then. These terms are particularly important in Multiple Choice Questions because they may change the verb or the whole sentence completely.
One website which is very useful for listening texts is MedlinePlus - be aware that the accent is American accents. This is a reliable source of medical information. If you look at the Videos and Cool Tools section you'll find some great practice resources. There are lots of videos and tutorials on diseases and conditions as well as surgical procedures. They all come with transcripts. I watched the video on 'Migraines in Middle Age' - the speed was quite fast, faster than the tutorial videos on the same site. So, great practice for OET. I searched for videos on 'bones' and found one by a radiographer who was explaining X-rays of the bone to a patient. Great for role plays!