try pronouncing it yourself and look at the shape of your mouth
ㅗ should make your mouth shape literally look like o (if you're practicing and overexagerating lmao)
ㅓwill kinda make your o look more oval.
now if you pay attention to natives speaking and you put a video on slow speed for example then look at the sound and the mouth movements combined.
usually ㅗ will sound like oh andㅓ more like aw and with time you will be able to hear it pretty clearly, i know this might not have been the clearest explanation im sorry 😭
but i really suggest listening to and watching lots of content and it will come naturally with time
knowing your vocabulary in general and how the words are written will also help immensely !!