As long as your F-1 OPT remains valid, you can work for multiple employers provided that all jobs are related to your major and all jobs are reported to your DSO. If you are on your STEM extension, each employer must also comply with employer reporting requirements.
But as soon as your H-1B change of status occurs, your F-1 OPT becomes invalid. That means that you can only work for the employer that petitioned you for H-1B status and must immediately stop working for all other employers. Then, those employers would have the option of petitioning for concurrent H-1B employment, which would allow you to start working for them again.
