It depends on what type of mulch and how deep the mulch is. In theory the fertilizer will be released onto the mulch then permeate down into the soil but you will lose a lot of fertilizer that gets caught up in the mulch especially if the mulch is coarse like wood chips. Instead I like to pull the mulch away from the roots of the plants, add the Osmocote fertilizer,  work it into the soil a bit then pull the mulch back on top of the soil. I have just made a video about this and it should be showing on the site soon. Keep growing, Marianne Binetti