My nerdlet has a few OCD tendencies. I think every toddler/preschooler has some, but my oldest has quite a few and some weird ones. For example, she won't go #2 and #1 in the same potty. She'll make you clean her potty before she goes again or she will sneak upstairs to use the other potty to finish. She did this even in diapers some and often would do the second function as soon as you changed her.
I've been told on a number of occasions by different professionals (therapists, doctors, teachers, etc.) to curb these because allowing them this young would only make them worse later on. I mostly try to do so. Some, like the potty issue, I don't know how to as I can't force her to go in the same potty and she can hold her need longer than I can. Others I'll admit are awfully convenient. She likes to clean the baseboards in the kitchen and will often beg me for a Lysol wipe to do so. I cannot pull out a wipe without her asking for one to clean something with.
I can actually tell how the day is going to go for the nerdlet in the morning by how much her compulsive tendencies show when she first gets up. The first thing she does on most mornings is collect all her ponies in a backpack to bring downstairs. She will also bring Mango, her flamingo. She always brings Mango and most mornings she will make sure she has at least her favorites of the ponies. This morning, she did not bring her ponies downstairs and I was sure that I was wrong about my theory because when I started listing the days activities, I got a lot of yelling 'no' and even an 'I need a break.'. Today, however, went extremely well despite the fact that we had a new therapist come to the house and a few activities out of the house. She actually did amazingly well and had her best Kindermusik day yet. On the one hand, my nerdlet's idiosyncrasies can be quite hard to deal with some days (especially if I'm having a bad day of my own), but on the other, I am glad that she gives me her own warning of the day that she is going to give me.
I sure do love this little girl and I hope that I don't end up turning her into a little version of the character Monk.