On June 1, 2015 OSHA issued a Best Practices Guide to Restroom Access for Transgender Workers. The Guidance can be found here.
In short, the Guidance states 
that employers should allow the employee in question to determine which restroom provides the “most appropriate and safest option for him-or herself.” The guidanc
e also provides some best practices, including that the employer allow, but not require, employees to use single-occupancy gender-neutral facilities and multiple-occupancy gender-neutral facilities with lockable single-occupant stalls. (I suppose we should be grateful that OSHA has not yet recommended multiple-occupant gender-neutral lockable stalls.)
The guidance further provides that employees should not be asked to provide any legal or medical documentation of their gender identity nor should they be required to use segregated facilities apart from other employees.
The guidance cites for support the fact that last month the OFCCP announced that under Executive Order 11246 it will require all federal contractors to allow employees to use restrooms consistent with their gender identity and the fact that the EEOC has taken the position that the negative reactions of other employees are no justification for an employer to require an employee to use any certain restroom. And, don’t think of converting your restroom into storage closets, OSHA standard 1910.141 generally requires employers to provide toilet facilities for all employees.
Keep in mind that this Guidance does not have the force and effect of law. It is, however, a good indication of the position that OSHA is going to take on this issue. Thus, employers must balance the interests of employees to use toilet facilities of the sex with which they identify regardless of their biological sex along with the wishes of other employees to use the restroom, whether it be in a locked stall or not, amongst persons of the same sex.
bswlaborandemploymentlaw.com 