Walker signed a two-year deal with Trinity ahead of the 2020 season after an impressive campaign in Super League with London Broncos the previous year.
Injuries have limited his time on the field at Wakefield, with the Scotland international making just 10 appearances.
He has spent the final months of the 2021 campaign on loan at Featherstone Rovers, helping them reach the Million Pound Game.
Greensmith is a product of Trinity’s academy and made his debut for the club against Salford in July, however that proved to be his only appearance for the club.
The outside back spent a large portion of the 2021 season on loan at League One Doncaster and has now signed a deal with Championship side Dewsbury Rams.
RULE CHANGES: Scrums are back and moves made to cut injury time-wasting in 2022“I am really pleased to be signing at Dewsbury and it was a straight-forward decision for me after speaking with Lee [Greenwood, head coach] about the club and his coaching techniques," said Greensmith.
“I think my time at Doncaster helped me develop physically as I had only played academy level before hand and it gave me the experience of playing in high-pressure games, which is what is needed to perform at Dewsbury.
“I am looking forward to the new challenges that come with playing at Championship level and I can not wait to prove myself to Lee Greenwood and the supporters.”
Rams head coach, Lee Greenwood added: “Ollie is a great prospect and we have done well to get him. I was surprised to hear that he was not staying at Wakefield and I am glad that he likes the sound of what we are trying to do here at Dewsbury.
“He has been a standout performer in an unfamiliar position while on loan at Doncaster and this has been his first taste of playing against men on a weekly basis.
“He is a big, strong lad that has the potential to be playing in Super League if he works hard.”