First Owner Occupied Multifamily
First, make sure your bank will finance your plan. If you want an FHA loan, your multifamily needs to be 2-4 units.
Next, make sure you look over all the leases before purchasing. When in doubt, have an attorney do this. You won't have cause for eviction simply because you want to live in a unit. On the other hand, if the tenant is month-to-month you can send them a letter to non-renew with notice appropriate in the lease (usually 30 days').
Explain in person to the tenant why you will be asking them to leave (e.g. you have to, in order to move in, in order to keep your mortgage on the place). Be polite. You might even offer to help them find a new place or move but you don't have to.
If they are on a month to month basis and you are in the process of purchasing you can even ask the sellers to ask one of them to move out just to get the ball rolling before you close on the property. If you're doing this in the lane county area a really good resource for this type of things is the Lane county Rental Owners Association. Some time when iv had questions about things iv called them and ran things by them to make sure everything was in the up and up.
Up are great examples given from our teammates. I like to always make it short and simple. Your question/s indicated that you guys have not purchased the property yet. If that is the case, when you are walking the property choose which side/unit of the property you would like to live in and when making the offer, have a contingency where the owner has to remove the tenants from that specific unit. You might need to help the owner of the property with ideas such as cash for keys or letting the tenants early of the lease without any penalties.