My 5yr old daughter was diagnosed with OCD at age 4 by her therapist. She too collects things. Mainly paper or toys around the house. She puts them in piles and in drawers or corners. If you remove things from her collection piles she usually has a complete meltdown. We have had allot of issues in the car as well. For instance her shoe falls off or she drops something, she starts kicking my seat and starts crying hysterically. She wants her little world "in order" and if it's out of order then she has meltdowns and aggressive behavior. She has also had a long history of behavior problems in daycares and has even been removed permanently. The best thing you can do is seek help now. Get him into a child counselor. They can spend some one hour sessions doing "play therapy" with him to better diagnose. She has taught me how to handle OCD. For instance don't take the child away from coloring or something they are in the middle of, instead tell your child they have one minute to finish. My daughter explodes if we remove her without warning. Set a timer and be consistant. The counselor says OCD isn't necessarily a bad thing as an adult. When they grow up to be an accountant it will be great. Just don't let it get extreme. Also OCD is brought on by anxiety, so take a look at his home/school environment. Also the counselor said 3-5 yr olds are still too young to diagnose with ADD or ADHD. I also believe my daughter highly sensitive to sugar. So at home she gets none but at daycare it's in just about everything. So try to limit the "bad" sugars. I am currently getting her tested by a naturopathic dr. as well.
I would also look at autistic spectrum disorders. Google DSM IV Autism and DSM IV Aspergers for the diagnostic criteria for both these disorders.
Does your son have age appropriate language and social interaction.
Collecting things is not typically OCD. OCD relates to needing rituals and routines usually because there is an accompanying 'dread' that something will go wrong if you don't do things in the right order the right number of times. Both autism and aspergers also can have obsessions (interests which can be unusual and very intensive), as well as a need for routine.
What is the anger about. Is it due to change or asking him to stop doing something eg turn off the TV to come and eat dinner?