Welcome to TPT.
We all make mistakes. Osmocote tabs will be releasing ammonia for a while but your bacteria and plants will catch up within days.
The critical factor here is your pH, as most of ammonia would be in relatively harmless ammonium at lower pH. In any case, you have a number of tools to minimize ammonia's effect on your fish:
- add Prime daily until ammonia < 0.25
- keep up the water changes, maybe at 50% daily
- add as many cheap fast growing plants like floaters, ambulia, water sprite, hornwort, guppy grass and such.
Best of luck.