The versatile performer will switch to MeadowÂ Lane when his Pompey deal expires at the end of this month.
Oxlade-Chamberlain had been on month-by-month terms after his initial Blues contract ended this summer.
That enabled him to continue receiving clubÂ treatment during rehabilitation for the leg he broke in training in April.
However, he has now agreed to join the League Two Magpies, who have offered a more realistic chance of first-team football.
Oxlade-Chamberlain hadÂ faced Kevin Nolan's side in the second round of the FA Cup last season, while on loan at Oxford City. The non-leaguers lost 3-2.
His sole Pompey first-team appearance arrived as a substitute in a September 2015 Johnstone's Paint Trophy encounter at Exeter.
He was an unused substitute in four BluesÂ matches last term, including three successive League One fixtures in October 2017.
However, the son of Mark Chamberlain never added to his tally during two years as a professional.
Kenny Jackett opted to loan himÂ to non-league pair Poole Town and Oxford City last season to aid his development.
Oxlade-ChamberlainÂ was then recalled from non-league football in March with Pompey squad numbers devastated by injury.
Notts County Â finished fifth in League Two last year and were subsequently eliminated from the play-offs by Coventry.