Scotland

It will take just under 4 hours to get from Edinburgh to Inverness by public transportation. You can take the train or the bus; a transfer may be required.

From Oban to Edinburgh, a trip of a bit longer than 4 hours, you have three choices: bus (which may not require a transfer), train (changing in Glasgow) or a combination of bus + train (again changing in Glasgow).

You can explore bus and train schedules--currently just through June 27--on the website https://travelinescotland.com/. However, I'd suggest verifying the rail schedules on the https://www.nationalrail.co.uk/ website, because I observed some inconsistencies on rail schedules when planning my own trip in 2019. The NationalRail website has never let me down. It currently is displaying schedules through June 18.

You'll be able to save a lot of money on rail fares if you're in a position to buy your tickets soon after they go on sale, while bargain "Advance" tickets are still available. They're the first to sell out. On the trip to Inverness, the advance fare can be as low as 12.90 pounds; the regular base fare is 50 pounds. Just looking at the NationalRail website now, I see that the Advance fares may not show up immediately, so it's good to check ahead of time so you know what fare will probably be available, then watch for when Advance tickets for your date are loaded.

I'm not positive about bus tickets, but I think they may be somewhat less expensive if purchased early.

As of yesterday Advance train fares across Scotland are only available up until 21 May. They are not always being issued 12 weeks ahead. Note that train schedules are not fixed until advance fares are issued- beyond 21 May is indicative, there are lots of instances currently of changing schedules way inside the 12 week limit.
I am currently running with a case in Wales where schedules and fares were released, then withdrawn from sale, and now all trains that day have been removed from the national rail system, but they will return at some stage. That journey is in just 6 weeks time.

If travelling by train you should get 3 x Two Together Railcards to get 34% off your rail fares.

I bagged myself a brace of the £12.90 fares from Glasgow to Inverness at the weekend, with a further 34% off for a railcard, for a forthcoming day trip from Glasgow to beyond Inverness.

Oban to Edinburgh by coach is always a change at Glasgow into Service 900 (that bus runs every 15 minutes). The one exception is a fairly new route 978 which leaves Oban at 0920 on a very scenic route via Stirling and Edinburgh Airport arriving Edinburgh at 1323. The return leaves Edinburgh at 1700, arriving Oban at 2102.
The only routes with advance coach tickets are Glasgow to Edinburgh and Glasgow/Edinburgh to Aberdeen and Inverness. These are "code shares" with Megabus and the far cheaper tickets are on the Megabus website. not the Citylink website. All other routes have fixed fares.
You can also buy tickets on Citylink to Thurso for Orkney- that is a code share with Stagecoach Highlands- who actually run the buses.
On train and bus alternate services to Inverness from both Glasgow and Edinburgh require a transfer at Perth- on the coach this is at Broxden Park and Ride, not in the City Centre.

PS- There has been a recent case on this forum of someone who thought they could buy £1 coach fares for seniors. You can't. Those are for Scottish Over 60's only. Foreigners (which in this case includes the English) can not buy them.

Between Edinburgh and Inverness, take the train. It's much more comfortable as you can get up, walk around, stretch your legs, etc. The trip will be approximately 3 hours and 45 minutes give or take a few minutes depending on which train you take.

By the way an Edinburgh to Inverness Coach Ticket is £36.50 when run by Citylink, and weirdly £33.50 when run by Parks of Hamilton, contracted to Citylink, but the exact same coach is only £8.30 when booked through Megabus a few weeks ahead, or only £17.50 booked the previous day. The journey time is exactly the same as the train.

It might be difficult to get a vehicle that would fit 6 and their luggage. Then, there are some narrow roads, so public transportation is probably best.

By the way you are all unlikely to get the £12.90 fares on the train mentioned by @ acraven, as these are usually capacity controlled to 4 per train, then 4 in the next price tier.
So you could wind up having train tickets in 3 price tiers.

The capacity controls on the coaches are not publicised, but seem to be more generous.