I have AS and suspected pure-O OCD which is in the process of being diagnosed.
AS basically comes down to social difficulties and 'special interests'. AS special interests can be seen as a type of obsession, but the difference with OCD is that the autistic person enjoys their special interest and it is often productive and meaningful.
The OCD obsession on the other hand is unpleasant and associated with anxiety; certain compulsions or rituals may be necessary to relieve this anxiety.
Personally, my problem is getting 'stuck' on certain things. If it's something good/enjoyable/productive (e.g. learning obsessively about a particular topic), then I call it AS. If I get stuck on something unpleasant, such as ruminating about something in the past and re-living and re-analyzing it over and over, then I see that as outside the common experience of AS. Like I said, pure-O OCD is the closest description of what I suffer from, and whatever the label turns out to be, I definitely think there is overlap in the thought processes and underlying neurology.