I would not assume OCD at this point. I also am not certain that you would find something really helpful quickly if you did. Your child has a "having to do a thing with a thing" issue, and that can be associated with some things like sensory issues, autism spectrum conditions, anxiety/obsessive compulsive rituals--and some young kids are just much more intense about such things. Some things you could research would be these:
1) Find out about EI services in your area--they vary in quality, helpfulness, and attitude, but some people who didn't want to go that route are pleasantly surprised.
2) Learn more about childhood anxiety and OCD--even if your child doesn't have it, learning about it can help. I haven't found a resource I consider perfect, but I have read Tamar Chansky's books _Freeing Your Child from OCD_ and _Freeing Your Child from Anxiety_--I wouldn't rush in trying to apply stuff, especially with a young child. It makes more sense when a child can say "I want to work on this" She also has a website called worrywisekids.org The ocdandparenting yahoo group is also a good resource if you decide to use it.
One thing I did find helpful with my son's obsessive compulsive issues was to be clear on my own boundaries--for example, in your situation I might decide that I would no longer hold the other end of strings--perhaps we'd go through a process where I'd do it 2 times but no more, perhaps I'd hide them all (and if the child "has to" do the thing whenever the child sees the thing but doesn't appear to miss the activity if it's gone, I would hide them to), it all depends on what seems to be working for my child. If my child starts getting me to play a role in a compulsive ritual, it usually strengthens his intensity here. If my child is yelling at me for "not doing X right" then when possible I stop doing X.
3) learn about sensory issues, as some children who are really into doing a particular thing with a thing over and over have them. Raising a Sensory Smart Child, The Out of Sync Child, one of those. There was a thread recently with sensory stuff.
4) For a child with an autism spectrum condition, some of the things you describe would be considered stims or perseverations (depending on both the activity and who you ask!, but these are more like perseverations, possibly). Reading some good, positive info. about the spectrum might be helpful to you. In the heaviest tantruming years, I didn't know what I was doing or what I was dealing with, so I'm not sure what would help! If the string does seem to fulfill a certain purpose for the child--an alternative to hiding the string might be to have a box of strings of different types and lengths for the child to experiment with.
5) Another important thing is to be able to be calm in the face of your child's distress. Much easier said than done, but always more helpful than the alternative.