Most of the Giants assembled on Monday for the first Organized Team Activity (OTA) of the offseason, but there was one notable exception. Star wide receiver Odell Beckham Jr. was not in attendance, according to a person familiar with the situation.

The 10 OTA sessions that will take place over the next three weeks are voluntary. But it's no secret that coaches would prefer to have the entire team in attendance as the installation of the offensive and defensive playbook begins.

It's unclear why Beckham missed Monday's session or if he plans to attend future OTAs. The next OTA is scheduled for Tuesday and Thursday's OTA is the first session open to the media.

It's not known exactly how many of the voluntary workouts Beckham attended during the five weeks of the offseason program that preceded OTAs, but he was not a regular participant, according to a person familiar with the situation. He also missed informal workouts organized by quarterback Eli Manning at Duke University in early April because he reportedly had oral surgery.

Beckham does not have any bonuses in his contract tied to participation in offseason workouts. The three-time Pro Bowler is set to earn $1.8 million in the final season of his rookie contract. The Giants recently picked up Beckham's fifth-year option for the 2018 season, which is worth approximately $8.5 million.

After having his fifth-year option picked up on April 24, Beckham sent a series of tweets vowing to work "harder than I ever have, to be the best that I ever have been. I'm more motivated now then I have ever been. Words are only that, so its time for action. To the LOYAL Giants fans out there. I'm lettin u kno now, this will be somethin uve never seen."

Beckham's Los Angeles-based trainer posted a video on Instagram of the receiver working out on Monday night.

Giants general manager Jerry Reese challenged Beckham to "grow up" in January after the receiver punched a hole in a wall outside the visitors' locker room at Lambeau Stadium following the Giants' 38-13 loss to the Packers in the Wild-Card round of the playoffs. Reese said last month that he is confident that the 24-year-old Beckham got the message.

"I think he is a guy that hears what we are saying and like (co-owner) John (Mara) said, 'We are not worried about Odell,'" Reese said on April 20. "He is a young kid, he is growing up every day and we think that he is going to continue being a tremendous football player and a tremendous representative of our organization here."

The first mandatory offseason activity is the three-day minicamp from June 13-15.

Cornerback Dominique Rodgers-Cromartie was in attendance on Monday after missing the first five weeks of the offseason program. Manning and the team's other top wide receivers, including Brandon Marshall and Sterling Shepard, also participated in Monday's session. It's not known if any players aside from Beckham missed the session.

Rams defensive tackle Aaron Donald, who was selected one pick after Beckham in the 2014 draft, reportedly missed his team's first OTA on Monday. Donald and the Rams are reportedly engaged in negotiations for a lucrative extension.

Dan Duggan may be reached at dduggan@njadvancemedia.com. Follow him on Twitter @DDuggan21. Find our Giants coverage on Facebook.