The 20-year-old made his Spurs first team debut in October, and scored on his second appearance for the club, away against Galatasaray in a Europa League clash.
Lankshear was also sent off after being booked twice in that game, and in total has played six times across all competitions for Spurs.
The striker then joined West Bromwich Albion on loan at the end of January. He played 11 times in Sky Bet Championship for the Baggies, but seven of those games came from the bench.
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Lankshear, who goes into the U’s squad ahead of the Championship opener at home against Portsmouth on Saturday, told the United club website: “This is a really exciting opportunity for me to be joining a great club.
“I learned so much last season in the first team at Tottenham and West Brom and I’m ready to now kick on with Oxford United.
“I’ve had some really good conversations with the manager, and I feel like this is the right place for me to be.
“I love nothing more than scoring goals. I want to do my best to bring as many as I can this season.
“My game is about more than just goals though. I’m a player that has taken all the information from the top professionals I’ve worked with and I’m now ready to bring it together, work hard for the team and deliver for the club.”
U’s head coach Gary Rowett added: “Will is an athletic, strong, quick goalscorer and someone who we feel can make an immediate impact.
“He has an outstanding record at youth level with club and country. He knows the way to goal, and we hope he can add those qualities to our team.
“With Nik Prelec and Will added to the squad in recent days, we are really pleased with the work that has been done to strengthen the forward line for the new season.”
United head of football operations Ed Waldron said: “Throughout the summer, we have worked tirelessly as a recruitment department to identify the right strikers for this team.
“It was always our intention to bring two quality forwards to the club, and we are delighted to have followed Nik Prelec with this loan deal for Will.
“He is one of the most exciting young forwards in the country. To bring in someone of his talent and ability is fantastic for the club and we hope he can be an important player for us this season.”