Does involuntary laughter or a smile upon hearing mockery of religion constitute a sin, especially given that I suffer from obsessive-compulsive disorder (waswasah)?

Since you are afflicted with waswas (obsessive whispering), we advise you to close this door, as opening it leads to great evil. You might imagine that something is mockery when it is not. Mere laughter or smiling does not necessitate sin. So, turn away from all these waswas.