NoClassDefFoundError for OkHttpClient

The latest version of Piccasso use an older version of Okhttp, you need to use a new dependency

compile 'com.squareup.okhttp3:okhttp:3.2.0'
compile 'com.squareup.picasso:picasso:2.5.2'    
compile 'com.jakewharton.picasso:picasso2-okhttp3-downloader:1.0.2'

Example:

File httpCacheDirectory = new File(getCacheDir(), "picasso-cache");
Cache cache = new Cache(httpCacheDirectory, 10 * 1024 * 1024);

OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder().cache(cache);
Picasso.Builder picassoBuilder = new Picasso.Builder(getApplicationContext());
picassoBuilder.downloader(new OkHttp3Downloader(clientBuilder.build()));
Picasso picasso = picassoBuilder.build();
try {
   Picasso.setSingletonInstance(picasso);
} catch (IllegalStateException ignored) {
  Log.e(LOG_TAG, "Picasso instance already used");
}