The best and simplest way to open an MBOX file in Outlook is to convert it to a PST file first. Outlook doesn't natively support the MBOX format, so there's no direct option to open an MBOX mailbox without converting it or using another email client as an intermediate step.
While it's possible to use Thunderbird and Outlook together as a manual workaround, that method takes more time and can become difficult if the mailbox contains multiple folders or a large number of emails.
Another easier way is to use specialized MBOX converter software. I recommend using software such as Softaken MBOX to PST Converter that helps in converting MBOX files into Outlook PST format, ensuring the converted mailbox can be opened directly in Outlook. This method is normally easier because it does not involve several manual procedures.
After the conversion, open Outlook, go to File > Open & Export > Open Outlook Data File, and select the PST file. Before deleting the original MBOX file, take a few minutes to verify that all folders, attachments, and emails have been imported correctly. Keeping the original MBOX file until you've confirmed everything transferred successfully is always a good practice.