Apple Mail has no option to read OLM files. Apple Mail can read MBOX files only. Neither Outlook nor Apple Mail can export OLM to MBOX format. However, few third applications can. These applications are not free and are payable. There are several tools that offer the same service. Among them, I really liked the functioning of OLM Converter Pro. This tool by Gladwev was too fast at converting my emails. Moreover, the mail contents and the mail folders were in the right place too.