.oga

Files with the filetype .oga (previously .ogg) contain lossy audio encoded with either Vosbis or Opus audio codecs. oga itself is just a container format.

.flac is a container and a lossless audio format which is a much better alternative for permanent audio storage. .oga encoded using Opus is fine for phones with support for it.

Note: Some portable music player devices will play .oga files containing Vorbis audio but not oga files containing Opus audio. Be sure to check what formats the players you will be using support before encoding files to .oga audio containers.