The 36-year-old has scored five goals this term, the last of which came in November, and has fallen out of favour recently under new Pompey boss Richie Barker.
Connelly told Oxford's official website: "I am really looking forward to it. The club are in a very strong position and is set up for success and I want to play a part in that."
Chairman Ian Lenagan added: "We have been looking for an additional striker throughout January, with extra budget allocated.
"Oxford United have a great chance of promotion this year and this improves those chances."
Connolly could make his U's debut against AFC Wimbledon this weekend.
- Sports & Recreation
- Oxford United