ooVoo has a built-in automatic echo cancellation mechanism but an echo may occur when one or more people in the conversation are using a set of speakers instead of a headset. It is caused when sound from the speakers is picked up by the microphone.
If you hear an echo, you can try the following:
If you prefer to use a set of speakers instead of a headset, try to position the speakers in such a way that the sound is not aimed in the direction of the microphone. You can also lower the speaker volume on the set of speakers which is causing the problem. The echo can also be caused by other participants in the video call. Try having them mute their microphone to see if the problem is on their end. The echo cancellation is also affected by background noise around you (it works best when you're in a quiet setting).