The integrated management system (IMS) is intended to include more than one management system standard, but which standards are included in your IMS is up to you. For all of the standards that you have included in your IMS, your certification audit should include a certification audit of all the different standards. A certificate should be issued for each standard that is included, since not every IMS includes the same standards (e.g. one IMS may include quality and environmental, another may include quality and OH&S, a third may include IT safety and environmental, etc.)
So, for your example above, if your IMS included OHSAS 18001 then your IMS audit should have included certification to this standard and you do not need to do it separately. If you are adding an OH&S management system into your IMS where it was not there before, then you will need to do this implementation and then have the certification audit completed for the new OHSMS.
If you are only now adding an OHSMS, you should really look at the ISO 45001 standard as the OHSAS 18001 standard has been replaced by this new standard and will become obsolete in the near future with companies transitioning over. You do not need to maintain certification to both standards, nor should you.
For more information on integrating management systems, you can see the free whitepaper: How to integrate ISO 9001, ISO 14001 and ISO 45001, https://info.advisera.com/9001academy/free-download/how-to-integrate-iso-9001-iso-14001-and-iso-45001