The addition of experienced Taylor, 34, and young talent Spratt, 21, will add muscle and further international pedigree to an already exprienced back division.
Taylor is a senior Wales cap while Spratt has represented Wales at U20 level. Spratt will remain with the Ospreys until May 2010 on a three year contract whilst Taylor has signed a two year deal.
Ospreys Head Coach Lyn Jones was delighted with his double acquisition. He commented, "Jonathan has been a key member of the successful Neath side of the last few seasons and he is a player of great potential."
"He is able to play in a number of positions and his versatility, natural ability and athleticism will, I am sure, secure him a fine future as a professional rugby player. Mark is a hugely experienced player and younger individuals like Jonathan Spratt and Andrew Bishop will benefit from his presence in the squad.
"He is a consummate professional and will bring us solidity at centre during World Cup year. You need a blend of youth and experience in any squad and it's about getting the balance right. When you sign more mature players it's about ensuring they still have the ability on the field, but also that they bring the right kind of influence to the squad, and Mark Taylor firmly fits the bill on both counts."
The Ospreys have also confirmed that prop Duncan Jones has extended his contract with the Welsh region until May 2010.
Ospreys Director of Rugby Kevin Hopkins said, "Duncan has been a cornerstone of the Ospreys pack since our formation and securing the services of players like him is vital in terms of the continuity of the squad. Building a competitive squad is not only about signing quality new players, it is also about securing the talent you have within your current squad."
Jones also earned high praise from his Head Coach, who said, "Duncan has been a key part of everything that has been good about the Ospreys since day one. He has developed into a senior international and has many more years of top flight rugby in front of him.
"He would have been a valuable acquisition for any team, but Duncan is an Osprey through and through and it is unthinkable that he would play regional rugby anywhere else."