One thing I learned about ten years ago when I was purchasing my first helmet is that, within one manufacturer's product line, some helmets can fit one shape of head (round or oval) while other helmets from the same manufacturer can fit the other shape.
I had been reading about a particular helmet made by Boeri but I couldn't find it in any of the shops that I had visited. I was discouraged when other Boeri helmets I tried did not fit my oval head. Then I saw a page in a paper catalog from one of the mail order ski shops wherein the people running the shop had hit upon a great idea (from the consumer's point of view.) Many of the shop employees had tried on each of the styles of helmets which the shop stocked and they rated the fit of the helmets for head shape. This was GREAT!
As it happens, within the Boeri product line, some helmets fit better on round heads while others fit better on oval heads. The one I had been interested in, which I had not found in any shop to try on, fit better on oval heads. I ordered it and was very happy with the fit.
Unfortunately, this shop no longer publishes such information and I haven't seen it provided by any other source.
I'm currently using a Giro Fuse helmet. It doesn't fit my oval head as well as my old Boeri but the Giro fits well enough. The trade-off for a less-than-optimal fit is that it has a controllable ventilation system and is compatible with some nice Motorola ear-flap-mountable wireless headphones.
Remember to bring your goggles with you when you try on helmets. Then at least you'll know before you purchase a helmet whether or not you will have to plan to purchase new goggles.