Someone already pointed out that DO's selling point are SSD drives. There's also a difference in the underlying technology. OVH uses OpenVZ, which are containers more than VMs, whereas DO is based on KVM.
Yet Linode's non-SSD drives seem to beat Digital Ocean's SSD drives "in just about every area and not by a small margin" (http://irj972.co.uk/articles/VPS-disk-perf-cont)
I don't know anything about OVH's performance - all I'm saying is that having SSD drives doesn't necessarily mean superb performance - there are other factors involved.