Sync Outlook and Apple Calendars on Mac and iPhone

Hello, I have outlook on my MacBook Air and use it for my email (an @btopenworld account). I have recently left work where I used Outlook calendar all the time and now can't decide between using the outlook calendar and the Apple Calendar app. Neither of them synch with each other, not every invitation that comes in goes into both calendars and my Outlook calendar account can't be seen on my phone (whereas the Apple Calendar app can). Bit frustrated and know I'm doing the wrong thing. Spoke to someone at Apple store who said I'd need to download the BT openworld app to see my outlook calendar but that's yet another app! Just seems crazy to have outlook for email and the Apple app for calendar. I'd welcome thoughts! thanks. Fi