An Oval Chainring is WAY better for mountain biking, especially if you do a lot of stand up pedaling. If you're a masher, get an oval chainring!!! Standing pedalling with a an oval chainring is where an Absolute Black Oval chainring really shines, allowing you to have a smoother transfer of power and get a greater output with each peddle stroke. This will lead to you getting out of the saddle more and having more fun on your bike. Also if you have knee problems or if you're recovering from knee surgery, definitely switch to Oval... you're knees will thank you and you'll be able to get over the top of the pedal stroke much easier. Essentially, you won't have that achy / stuck / hard to turn the cranks over feeling at the top of the pedal stroke when pushing a higher gear.
For those of you who track your times on Strava, I will almost guarantee you will see a large improvement in your trail times (and probably have some PRs too without really trying) when you switch to an Oval chainring, and the effort will feel much less. The very first ride using an AB oval 34t chainring, I knocked over 2 minutes off a Strava segment where my previous best time was 9:44 min/sec (10 rides), I did the segment in a time of 7:43 min/sec (and I wasn't trying super hard, this was after only 5 minutes of using the oval chainring for the very first time). This isn't unusaul either, if you read other reviews from riders who have switched to an oval chainring.
So with that said, I highly recommend Absolute Black Oval Chainrings for mountain biking, I ABSOLUTELY love using the 34 tooth chainring! And I'm not alone, just check out all the Absolute Black positive reviews on ChainreactionCycles.com... Once you switch to oval, you'll probably never go back to using a round chainring for mountain biking again. Honestly, Absolutely Black Oval Chainrings are the TRUTH... it was pretty mind blowing the first time I used one. If you want to be a faster mountain biker, especially on the up, go Oval!!! Also if you like a bigger chainring on the front (such as 34t or 36t), an oval chainring should be much better and easier to pedal!
That being said, if you're a rider who sits down and pedals the majority of time and spends the majority of time sitting and spinning a small chainring, and larger cog in the back, you may not find the difference of an oval chainring as great when compared to a round chainring. But I'd still suggest giving an oval chainring a try, as we think the benefits of oval chainrings truly outweigh round chainrings.
If you want a more rounded oval chainring as compared to the AB chainring, then you might want to check out the Wolf Tooth oval chainring.