Old Bay seasoning and Cajun Seasoning are similar and used commonly in the same types of dishes to season food.
Cajun Seasoning does make a good substitute for Old Bay seasoning but they have a pretty different taste but work well in the same dishes and on the same food as a substitute.
Unlike Cajun Seasoning the Old Bay seasoning uses and layers in some more exotic flavors, like allspice, nutmeg, cloves, and cardamom.
So Old Bay has a better flavor to it than Cajun Seasoning but you can certainly use Cajun Seasoning instead of Old Bay Seasoning when you want to or need too.