If you have already implemented another standard such as Iso 89001:2015 or Iso 14001:2015, then you will have implemented most of the new requirements for ISO 45001:2018 standard (such as context of the organization and interested parties) since the main changes in the standard have come from the re-formatting to Annex SL. Apart from this the most important thing to review are the requirements for consultation and participation of workers in the standard as this is also new.
One of the best ways to ensure that you have everything is to perform a gap analysis. You can use a simple gap analysis such as this free ISO 45001 Gap Analysis Tool (https://advisera.com/45001academy/iso-45001-gap-analysis-tool/), or you can take each ‘shall’ statement of the standard and place this is a table with a column to assess if you already do this and where the information is located. You can then find what you are not doing, which are th e gaps that you need to address.
For some help planning the transition from OHSAS 18001 to ISO 45001 see the whitepaper: Twelve-step transition process from OHSAS 18001 to ISO 45001, https://info.advisera.com/45001academy/free-download/twelve-step-transition-process-from-ohsas-18001-to-iso-45001