OSHA rules apply to tobacco smoke only in rare and extreme circumstances, such as when contaminants created by a manufacturing process combine with tobacco smoke to create a dangerous workplace air supply that fails OSHA standards. Workplace air quality standards and measurement techniques are so technical that, typically, only OSHA agents or consultants who specialize in environmental testing are able to determine when the air quality falls below allowable limits. However, many state and local governments have separate laws that prohibit or restrict smoking in the workplace.
For more information, see Nolo's article Smoking in the Workplace.
For a comprehensive guide to workplace laws, get Your Rights in the Workplace, by Barbara Repa (Nolo).