Occupational stress is the kind of stress that comes with the work, but not necessarily with the job. For example if you are an electrician, you know the risks and the necessary effort it takes to do a specific task. Since you prepare for that, this kind of occupational pressure does not generally turn into stress.
However, if you don't get the necessary materials or your clients start pestering you (which might happen as part of your job), such environmental stress might turn what is otherwise regular work into a stressful activity as well. You might be also be worrying about a sick family member, and that might distract you from work enough to make it become a stressful activity.
Of course, some occupations are more about risk and stress management than others; if you have to deal with people intensively you'd better leave a large margin for error and learn not to get stressed when things go wrong.
When occupational stress is within boundaries, it's generally a good kind of stress - there's a sense of accomplishment for work well done. If you feel your control slipping, take a break, recollect your thoughts and try to get rid of unnecessary distractions.