The 22-year-old joined the Czech First League from IF Elfsborg in 2025 but has spent the majority of his time at the club out on loan.
Ouma turned out for Lech Poznan during two spells over the past two seasons, lifting the Polish League title in the recently completed campaign.
His move to the Addicks includes an option to sign on a permanent deal.
Chalton manager Nathan Jones told his club's website: "Timothy is a player that we've monitored for a long, long time, and he comes with a real good pedigree.
"He's an athletic, technical midfield player who has had some good experience, and he'll add real quality to our midfield."
Ouma is the Addicks' seventh signing and joins fellow new additions Ivan Mesik, Billy Koumetio, Danny McNamara, Nathaniel Chalobah, Millenic Alli and Karlan Grant at the English club.
Lens produced a remarkable late comeback to beat Slavia Prague 3-2 in a five-goal Champions League thriller, scoring twice in stoppage time to snatch victory from the Czech side on Thursday.
Kenya Sports Cabinet Secretary Salim Mvurya has challenged Benni McCarthy to lead the Harambee Stars to Africa Cup of Nations glory.
Tottenham Hotspur gave Roberto De Zerbi plenty to smile about as they cruised past Charlton Athletic 5-1 in the Carabao Cup, with new signing Savio marking his debut with a goal.
Hosts Morocco and Algeria have both secured their places in the quarter-finals of the Women's Africa Cup of Nations after the final round of Group A matches.
