Oshima sits at the bottom of the drivers' standings heading into this month's season finale at Suzuka, as the only driver in the 21-strong Super Formula field to have not yet scored a point this season.
His best result of the year so far has been 13th in the July Fuji race, while he has also failed to progress from Q1 all season.
Speaking to Motorsport.com, Oshima admitted Rookie had failed to make any real progress despite some encouraging signs earlier in the year.
"At the beginning of the season, it felt like the car was improving and things would get better," he said. "But since [Round 5 at] Sugo the car has felt strange, lacking grip, and big tyre degradation, so we are struggling again.
"The team has been working hard to try and find the problem, but still we cannot find it. We went back to the set-up we had at the beginning of the season, but the feeling is not the same.
"Already we have started talking about next year with the team, and we need to find some solution. I have to think a lot."
Oshima, 35, made his debut in what was then known as Formula Nippon in 2009 for TOM'S and has a single win to his credit at Sugo in 2010.
However, he hasn't finished higher than eighth since joining the newly-established Rookie Racing squad in 2020, with the current campaign marking the team's second as an independent outfit.
Asked directly if he wishes to continue in Super Formula next season, he replied: "I don’t know. It’s very difficult to say."
Rookie Racing is the only one-car team in the Toyota stable, and Oshima admits this has also been a major hindrance.
"This is a problem both set-up-wise and driving-wise," he said. "I don’t know if it’s a car problem or it’s my problem."
However, Oshima downplayed the prospect of Rookie being able to add a second car in time for next season, commenting: "There are not enough people, so we need more time."
The arrival of former TOM'S engineer Tsutomo Tojo has also not made the hoped-for difference to Rookie's fortunes, according to Oshima.
"He knows what set-up TOM’S used to use, and we put almost the same on our car and it’s not working," he said of Tojo.
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