Is nucleolus an organelle?

Yes, the nucleolus is an organelle that is present as a distinct structure in the nucleus of eukaryotic cells. The nucleolus is the largest nuclear organelle and is composed of RNA and proteins that form around specific chromosomal regions. 

The nucleolus serves specialized functions. It plays a critical role in ribosome biogenesis, synthesis of ribosomal RNA molecules, and alteration of transfer RNA.  It is also responsible for transferring ribosomes and ribosomal RNAs out of the nucleus into the cytoplasm, where protein synthesis takes place.