Abstract Background Evidence shows that clinician-delivered brief opportunistic interventions are effective in obesity, and guidelines promote their use. However, there is no evidence on how clinicians should do this, and guidelines are not based on clinical evidence. Purpose A trial (Brief Interventions for Weight Loss [BWeL]) showed that brief opportunistic interventions on obesity that endorsed, offered, and facilitated referral to community weight management service (CWMS) led to 77% agreeing to attend, and 40% attending CWMS, as well as significantly greater weight loss than control at 12 months. We assessed which behavior change techniques (BCTs) doctors used that were associated with CWMS attendance. Methods We coded 237 recorded BWeL interventions using the behavioral change techniques version one taxonomy. We also coded the BWeL training video to examine delivery of recommended BCTs. Mixed effects logistic regression assessed the association between each BCT, the total number of BCTs, and delivery of recommended BCTs, with patient’s agreement to attend and actual CWMS attendance. Results Of 237 patients, 133 (56%) agreed to attend and 109 (46%) attended. Thirteen BCTs were used more than eight times but none of the 13 were associated with increased attendance. One, “practical social support,” was significantly associated with increased patient agreement (odds ratio [OR] = 4.80, 95% confidence interval [CI] = 1.15, 20.13). Delivery of recommended BCTs and the total number of BCTs used were both associated with increased agreement (OR = 1.56, 95% CI = 1.09, 2.23 and OR = 1.34, 95% CI = 1.03, 1.75, respectively), but not attendance at CWMS (OR = 1.20, 95% CI = 0.98–1.47 and OR = 1.08, 95% CI = 0.94–1.24, respectively). Conclusions There is no evidence that particular BCT can increase the effectiveness of brief opportunistic interventions for obesity in adults. However, using more BCTs and delivery of recommended BCTs may increase agreement to attend community weight management services.