Basic life support (BLS) is one of the most efficient ways to improve out-of-hospital cardiac arrest (OHCA) victims’ outcomes. Resuscitation initiated by a random witness of OHCA is essential to increase the chances of survival. To assess the impact of BLS training in cardiac patients on knowledge about first aid for OHCA. The study group consisted of 68 participants who completed a questionnaire prior to BLS training. Forty-three of them then filled out the same questionnaire again after the BLS course. Participants’ knowledge was assessed with a self-designed questionnaire, which comprised 41 questions divided into six domains, namely legal aspects, resuscitation technique, resuscitation algorithm, knowledge about using an automated external defibrillator (AED), “calling for help” knowledge and identifying sudden cardiac arrest. The average score before the BLS course was lower compared with final results (43.8% ± 15.6% vs. 68.6% ± 22.7% [% of max. score], p = 0.001). The best scores, both before and after the BLS course, were gained in the “calling for help” knowledge (79.5% ± 33.5% vs. 80.4% ± 17.4% [% of max. score], p = 0.5) and “knowledge about using AEDs” domains (62.4% ± 35.2% vs. 74.7% ± 29.3% [% of max. score], p = 0.1). Patients who completed first aid courses gained better scores in the “knowledge about using an AED” domain (93.3% ± 14.9% vs. 58.6% ± 35.4% [% of max. score], p = 0.02). No differences between the other domains and overall scores were reported (total score: 48% ± 12% vs. 42% ± 17.5% [% of max. score], p = 0.5). General knowledge about BLS is poor. BLS training in cardiac patients improves knowledge about first aid for OHCA. Education and hands-on training are crucial to improve outcomes.