One quick way you can do this is to just keep breaking it down sufficiently so you know what the next thing you have to do is. Say you have an action today (3/14) to memorize the first 1000 digits of pi, and you're working on it when the phone rings. The caller is your neighbor reminding you that the two of you were going to fix your fence today. You've only gotten the first 50 or so digits securely memorized, so you go to that action (in project/planning mode) and do Edit->Outlining->Add Child (cmd-}). Type in your next action -- "memorize digits 51-1000" and go off to your other task, knowing that you can resume this one at the right spot when you return to it. You can repeat this procedure recursively, as many times as needed. You can use OmniFocus' arsenal of task management tools (start dates, due dates, flags, etc.) to schedule the execution of the remaining work, if desired.
It's a useful tactic with tasks which might have multiple steps, but are routine (so you don't need to plan it out) and you would normally do all together if not interrupted. When I have such things where I do expect to be interrupted, or might not remember all of the steps accurately, then I use Curt Clifton's
Populate Template Placeholders script to stamp out a checklist project to make non-atomic completion of all the steps more reliable.