Oath-Helping. R. v. D.M.
In R. v. D.M. (Ont CA, 2022) the Court of Appeal comments on the use of expert testimony on credibility:
... There are significant limits on the contribution that expert witnesses can make to credibility assessments, including the rule against oath helping, which prohibits the reception of expert opinion about the truthfulness of a witness: R. v. J.H., 2020 ONCA 165, at para. 94; R. v. Llorenz (2000), 2000 CanLII 5745 (ON CA), 132 O.A.C. 201 (Ont. C.A.), at para. 27. ... . R. v. Oppong
In R. v. Oppong (Ont CA, 2021) the Court of Appeal considered oath-helping:
 The determination of a witness’s credibility and reliability is for the trier of fact and is not the proper subject of expert opinion. An expert opinion cannot be introduced if the purpose is solely to bolster a witness’s credibility (i.e.: oath-helping). “Credibility is a notoriously difficult problem, and the expert’s opinion may be all too readily accepted by a frustrated jury as a convenient basis upon which to resolve its difficulties”: R. v. Marquard, 1993 CanLII 37 (SCC),  4 S.C.R. 223, at p. 248.. R. v. Santhosh
In R. v. Santhosh (Ont CA, 2016) the Court of Appeal canvasses the law of the evidence rule against 'oath-helping':
(i) The Rule Against Oath-Helping
 The rule against oath-helping bars parties in most circumstances from introducing evidence solely to support a witness’s credibility. In R. v. B. (F.F.), 1993 CanLII 167 (SCC),  1 S.C.R. 697, at p. 729, Iacobucci J. stated the rule in the following manner:
The rule against oath-helping prohibits a party from presenting evidence solely for the purpose of bolstering a witness’ credibility before that witness’ credibility is attacked. This type of evidence is of the sort that would tend to prove the truthfulness of the witness, rather than the truth of the witness' statements. Evidence that might have an incidental oath-helping aspect is admissible when tendered for some other admissible purpose. That said, “where evidence is admissible for another purpose, but it is also oath-helping, a court should take any appropriate steps necessary to limit the oath-helping nature of the evidence, including cautioning the jury”: R. v. Mallory, 2007 ONCA 46 (CanLII), 220 O.A.C. 239, at para. 280. Trial judges sitting alone should heed this caution and use such evidence only for the other admissible purpose. In this case there was no other admissible purpose for the evidence.
 Consequently, the trial judge could not use the evidence of the complainant’s religiosity for credibility purposes, unless some exception to the rule against oath-helping applied. There are two primary exceptions to the rule against oath-helping, neither of which apply here.
 First, the rule against oath-helping does not apply to an accused person testifying in a criminal proceeding: R. v. Clarke (1998), 1998 CanLII 14604 (ON CA), 129 C.C.C. (3d) 1 (Ont. C.A.), at para. 20. The law gives wide latitude to accused persons to introduce oath-helping and other character evidence.
 Second, a party can introduce oath-helping evidence to bolster the credibility of a witness whose credibility has been attacked. However, the scope of permissible oath-helping evidence in such a situation is limited. In R. v. Tash, 2013 ONCA 380 (CanLII),  O.J. No. 2642, Watt J.A. helpfully laid out the interplay between an attack on credibility and the evidence that may be adduced in response to it:
 A witness’ character for truthfulness or mendacity is relevant circumstantial evidence on the question of the truthfulness of the witness’ testimony. Evidence of a witness’ previous deception tends to demonstrate a character for untruthfulness. In turn, the existence of such a character trait increases, at least slightly, the probability that the witness has lied under oath. Proof of a witness’ character trait for untruthfulness can be accomplished in several ways including proof of prior untruthful conduct, the witness’ associations, and prior history. Any other acts offered to establish character should have a significant relation to credibility.
 Our adversary system requires that the proponent of a witness be afforded an opportunity to meet attacks on the credibility of the witness by presenting evidence rehabilitating the witness. But the bolstering evidence must be responsive to the nature of the attack and not exceed permissible limits. For example, supportive evidence of good character for honesty of a witness impeached by evidence of “bad” character for untruthfulness or dishonesty is permissible. Proof of prior consistent statements to rebut impeachment on grounds of recent fabrication is also permissible. At root, the admissibility of rehabilitative evidence should depend on whether what is proposed is logically relevant to rebut the impeaching fact. The rehabilitating facts should meet the impeachment with relative directness. The wall, attacked at one point, may not be fortified at a distinctly separate point. [Citations omitted; emphasis added.]