As of today, 3/31, Alaska Airlines has officially joined Oneworld, When this news first came out last February, we briefly covered it here: Alaska Airlines Announces It Will Join Oneworld
Oneworld members + all partner airlines
Although Oneworld is arguably the weakest of the three major alliances, it still has several high-quality members, including newly added Alaska, Japan Airlines, Qatar Airways, and Cathay Pacific.
As an example of reciprocal mileage earning, here’s a look at earning between AS and AA:
Fly AS and credit to AA:
Fly AA and credit to AS:
Looking at these charts, it’s clearly more attractive to fly AA and credit the miles to AS.
Finally, let’s look at all of AS’s current partner airlines: 
The overall quality of these partners is very high, but many of them come from other alliances, including SkyTeam and Star Alliance members such as Singapore Airlines, Korean Air, and LATAM... Now that AS has officially joined Oneworld, it’s very possible that some of these partnerships will become less extensive, or even end in areas like mileage earning and redemptions.
Elite benefits
One of the biggest concerns for AS elites and Oneworld elites is, of course, elite benefits.
First, here’s how AS elite tiers map to Oneworld status:
MVP = Ruby, MVP Gold = Sapphire, MVP 75,000 = Emerald
Using an AS member flying AA as an example:
MVP members can select Main Cabin Extra seats for free within 24 hours of departure.
MVP Gold and 75K members can select Main Cabin Extra seats for free at the time of booking.
MVP Gold 75K members can receive complimentary First Class upgrades at check-in on flights within North America and Central America, and may bring one companion.
Note that all benefits are subject to availability.
Using an AA member flying AS as an example:
This depends on fare class, but it’s still pretty solid.
In terms of benefits, I think the three most appealing ones are upgrades, lounge access, and free checked baggage. Those are probably the key points most readers should focus on.
Summary
My view is neutral, but slightly negative. Why?
AS’s own MVP Gold and 75,000 members have had extremely high upgrade odds on Alaska-operated flights. I’d even say that for 75,000 members, outside of SEA and PDX, the odds of getting upgraded to First Class can exceed 80%. That’s because AS has far fewer elite members than a giant airline like AA. But now that alliance elites can also compete for upgrades, those 4/6/8 pseudo-First-Class seats on a 737 are going to be much more competitive.
On the other hand, AS elites now get the chance to try their luck for upgrades on Oneworld carriers like AA, though the odds are almost certainly lower than what they previously enjoyed on Alaska’s own flights.
For AA and other Oneworld elites, though, this is definitely a bigger win. Previously they didn’t get much treatment on AS; now they effectively receive something much closer to their home-airline benefits.
One more point: AS miles have historically been highly valued, and devaluations had been relatively limited. But now that Alaska has joined Oneworld, there are more redemption options and a larger overall network, so a devaluation is probably only a matter of time. Last year I said it would happen within two years; there’s still one year left on that prediction, and I’m willing to bet on it.
Related articles:
The Alaska Airlines Inflight Meals I’ve Had Over the Years
From the West Coast to the East Coast (A Practical Alaska Airlines Points Redemption Example)