OCD behaviour?

[CHST]

My son is 3 1/2 and for the most part a very lovely boy who loves to hug and is very affectionate. However recently I have noticed some behaviour which I am questioning whether is normal toddler behaviour or something that needs investigating.
He is very strange around the bath...we will have times he is okay but he hates washing hair and will not get in the bath at all if there is a bit of fluff or a hair in the water. I have to take it out and then he will meticulously scour the bath for more "fluff" often this results in a missed bath as he will get really upset if you try and make him get in.
Another behaviour which doesn't occur every day but has quite frequently is he will not go toilet until bursting. He has to remove all his clothes on the bottom before getting on but by this point, he is often so bursting that the wee ends up going on the seat, his legs and the floor. He then has the most enormous melt down because he needs "bigger wee." We have worked out that he wants the wee to go in the toilet and not the floor but by this point it is impossible. He runs around screaming his head off and is inconsolable. I have tried many different things but nothing works, have tried to hold him but physically can't and this makes him worse, have tried distracting by mimicking behaviour, tried being stern but nothing works and he will run around absolutely distraught for about 30 minutes until at some point we have convinced him to get back on the toilet in an attempt to get more wee out.
His speech is fine by the way but something else I have noticed is that he loves cars and trains and at preschool, it is very difficult to get him to do anything than play with trucks and cars.
I just don't know how to deal with the screaming behaviour and if it is something that needs to be talked about with a professional.

First thing I would find out is whether or not he behaves the same way regarding toileting in school. If he is happy to go at school 'normally' without meltdowns then I would say tackle this behaviour at home without worrying there might be an underlying developmental problem. It may be that, having had one bad experience (of getting horribly wet etc at home) then he has developed a bit of a fear of your toilet (hence the removal of clothing as he fears getting wet). Stick him in clothes that are easy to remove, throw some cheerios in the loo and make a game of aiming at them. Don't force him back to the toilet when he thinks he is done - you will only make it worse and turn it into a battle for control. Since it is his body and his wee, ultimately he will win this battle! :) See if sitting to wee is easier for him just now instead of standing up.

Re bath times - many children hate getting hair washed even at this age. My eldest hated it until well past her 4th birthday. Give him a cloth for his eyes or get him to help put the water on and rinse. Try one of those visor things which keep water out of eyes and nose. The fluff hunting thing - again, many children develop convoluted regimes and rituals without being ocd or on the autistic spectrum. Some of it may well be him playing power games and manipulating as he doesn't want a bath and this successfully prevents one happening. Take him up 30 minutes before bath time to 'clean the bath' with you, choose toys to play with, get his bed time story book etc. Let him have his cleaning as part of the routine, but you control it so he is not controlling you. If he refuses to get in the bath tell him nothing else is happening (story, cuddles etc.) until he does. If it is behaviour to control you and he is strong willed you may well have a fight on your hands as he digs his heels in, so choose a night where he can be late to bed and it won't matter the next day.

Lots of children develop obsessions with toys, princesses, cars, space, lego etc. He sounds perfectly normal to me. If you are happy with other areas of development - eye contact, accepting physical contact etc. - then I would try to avoid worrying about behaviours being a symptom of an underlying 'issue' ' and try instead to look at the reasons for each behaviour.