Boy, OCD is pretty ********s isn't it? I'm sorry you're suffering so much. I definitely relate. My OCD onset in 2000, and in 2001 I went through a period where I worried obsessively about my parents' death.
I think recognizing that your thoughts are rooted in OCD is a good place for starting to deal with them. Therapy is always good as well, as they don't call it "the talking-cure" for nothing. It sounds like you've had very real tragedy in your life before, and fearing more seems reasonable all things considered.
Death is a fact, but you're right that you should absolutely be able to choose to enjoy the time you DO have with your parents. And there really is a choice there, odd as that may sound. OCD robs us of choice in strange way. By electrifying us with anxiety, it takes away joy. But if you can recognize your OCD thoughts for what they are you can let them exist instead of resisting them, and your response will lessen in intensity with time. Have you read any books on OCD? Have you been to therapy or been on meds? Just a few thoughts... Let me know. I'd be happy to dialog.
You are not alone, friend.
Captain OCD