Long-time Toyota driver Oshima is going into his fourth season with the one-car Rookie squad, which failed to score points in 2022 and finished at the bottom of the teams' standings.
However, the introduction of Super Formula's new SF23, which produces less downforce than its SF19 predecessor, has seemingly boosted the 35-year-old's prospects heading into the new campaign.
Oshima topped the morning session on the second day of this month's sole pre-season test at Suzuka, setting the seventh fastest time that day and the 12th-fastest across the two days.
While many drivers have reported the new SF23 has more tendency to oversteer, Oshima said he felt more stability at the rear, something for which he doesn't have a clear explanation.
"The car finally feels ‘normal’," Oshima told Motorsport.com. "Until last year there was something strange, but now we have less downforce [with the SF23], I actually feel more downforce!
"I think my car is much more stable than last year. It’s something strange. I feel more grip at the rear than before, but I don’t know why."
Rookie's lack of results last year prompted Oshima to admit he was struggling to decide if he should continue racing in Super Formula, in which he first made his debut back in 2009 following a one-year spell in Europe.
But now Oshima says he is relishing the prospect of a more competitive year.
"I have been last for a lot of seasons, but this year I need to get more motivation back," he said. "I am looking forward to it."
Rookie Racing has a new team director this season in the form of two-time Super Formula champion Hiroaki Ishiura, who replaces Tatsuya Kataoka in the role.
Oshima won the 2007 GT300 title in SUPER GT alongside Ishiura, and the pair spent another two years sharing a cockpit in the GT500 class in 2009-10.
Ishiura backed Oshima to make a comeback this season, pointing out that he was a similar age when he returned from the Super Formula wilderness to claim his two championship titles in 2015 and '17.
"The car has been working well since we started running [in testing], and so the atmosphere in the team is likewise improving," Ishiura told Motorsport.com's Japanese edition.
"I came from not having a seat for two years to joining Cerumo/Inging [in 2014], and I experienced being in a team where, if you produce your best performances, you can become champion.
"I have seen up close how good Oshima is, and I know that if he gives his best performance, we can be fighting towards the top. I want to support him so he can show his real level."
Oshima also pointed out that Ishiura has plenty of experience of the SF23 from his time as Toyota's development driver for Super Formula's new car last year, a role he has now relinquished.
"He doesn’t know any specific set-up numbers, but he knows the important parts to find more speed," said Oshima of his former SUPER GT team-mate. "We have a lot of good ideas, and we had a good test."
