Scotland Inverness to Oban

If you look at the Scotrail map, you’ll see that there is no direct train service between Inverness and Oban, so unless you want to go down to Edinburgh and Glasgow and thence back up to Oban, you’ll be using buses. If you search for your journey online, you’ll find the bus options - Citylink have a service but there might be others too.

I know nothing about this website, but I was searching for a joint map of Scottish public transport, and this came up:
https://busandtrainuser.com/2024/08/24/enjoying-the-spirit-of-scotland-part-1/

There is no map of all Scottish public transport. For one reason it is too dense to map. Secondly the network is currently changing so fast (for the better) that no map could keep up.
The most sensible way is Scottish City link #919 hourly to Fort William, changing to the twice daily (not Sunday) #918 to Oban.
City link show three other services where you change additionally at Tyndrum.

Alternatively take the very scenic train from Fort William to Oban changing at Crianlarich.

I am half expecting a new additional route by bus to be announced in early summer, but time will tell.

This website can be a useful resource for trip planning...

https://www.travelinescotland.com/cms/home

I contacted West Coast Motors and was told that I can book the 919 from Inverness to Fort William but the 918 to Oban is a "walk-on." We will be there in July. Does anyone have experience with this? What are the chances that we won't get seats and will be stranded in Fort William?

No chance, the 918 is both a local service and a long distance service.
It has to be able to pick up on demand at lots of remote stops.
As the latter it is bookable either as a standalone journey or as sector 2 of a through journey from Skye or Inverness through citylink.

WCM run many City link services under contract including the Glasgow to EDI airport shuttle.