After being delisted by Melbourne in 2020, McDonald was granted a lifeline by the Blues, who handed him a one-year rookie contract for 2021.

Unfortunately, he was limited to just three games as the substitute due to a back injury and only took the field once.

He still did enough to earn a new deal.

"I've sorta known for a fair while," McDonald said on his re-signing.

"More or less, it was reliant on my medical check as I've been injured all year.

"It's good that they have the faith that they can get my body right and I can get my body right."

The injury was diagnosed as a lower back stress fracture and plagued McDonald for most of the year.

He is confident that he can remedy it for 2022.

"Whilst your in season you've got the itch to play and try and get back quicker than what you probably should," he said.

"We've let it heal properly and put together a good plan."

During his brief playing time in 2020, McDonald was used predominantly in the forward line and in the ruck.

In the round one loss to Richmond, a highlight came when he replaced Jack Silvagni in the third quarter.

The former Edenhope-Apsley footballer gave the Blues a much-needed spark by kicking two goals - twice as many as he kicked at Melbourne in 81 games.

Despite the impressive performance, McDonald doesn't expect that his forward line heroics will translate to a permanent move.

"It was outlined from the moment that they wanted me to train on, that they saw me as a key back," he said.

"I don't mind getting swung forward every now and again. It's good fun."

While one might think McDonald would be looking to put 2021 behind him pretty quickly, that couldn't be further from the truth.

After his Melbourne exit, the 25-year-old wasn't expecting another chance at the highest level.

"To be honest it was one of the best years," he said.

"After I was delisted from Melbourne, I pretty much had work lined up and had a local football team that I was going to play with.

"All of a sudden, I got the call to come and train on, so anything extra was a bonus.

"When I played the practice match and then the NAB Cup match, I was over the moon just to be there."

McDonald's wasn't the only signature that the Blues secured in recent weeks.

During the AFL trade period, the side added some quality to their midfield in former-Swan George Hewett and former-Docker Adam Cerra.

Carlton finished a below-expected 13th in 2021 and will be hoping their new additions can help them crack the top-eight in 2022.

In the meantime, McDonald's focus will be on getting his body right and returning to proper training with the group.

"I haven't ran for three and a half months," he said.

"I'll start doing very basic running in the next month and then try and get re-introduced to the main group by that Christmas stage.

"That's the initial goal and after that it's just to try and string some games together and win some games. But we've got a long way to go before that happens."

Oscar's re-signing wasn't the only reason to celebrate for the McDonald family over the past month.

His sister Sacha signed for the London Pulse netball side, while his older brother and ex-teammate Tom was part of the Demons' side that saluted on the grand final day.

It is almost a given that a player would feel mixed emotions towards a successful team that recently delisted them.

But when asked, McDonald's voice just exuded with pride towards his brother.

"It was amazing," he said.

"I'm so proud of Tom because the two years prior, he was not down and out, but Melbourne wanted to trade him, and he probably wanted to get out of Melbourne.

"They wiped the slate clean and made some changes within the club, and Tom played a pivotal role, especially in the front half of the year and during the finals series.

"He's achieved such a big result from where he was to where he got to in the year.

"Everyone was so proud of him."