Oyeleke, who can also play at full-back, had spells at the academies of both Chelsea and QPR before joining Brentford, where he has made two first-team appearances.
The 20-year-old will be added to the Northampton squad for their trip to Gillingham on Tuesday night.
Cobblers boss Aidy Boothroyd said: "I have watched Manny play a number of times and he will add to our squad.
"He is a strong, athletic, combative midfield player who can play anywhere across the midfield or at full-back.
"He is here to challenge for and compete for a first-team place and to gain experience. I think we can help Manny and I know he can help us."