228 Background: Serious illness conversations (SICs) are discussions between oncology clinicians and patients with cancer about illness understanding and care preferences. SICs are associated with less intensive and more goal-concordant care near the end of life, but many patients die without an SIC. Methods: PATH-SIC is an ongoing, single-center randomized trial (NCT05629065) that enrolls adults without a documented SIC starting a treatment for breast, gastrointestinal, genitourinary, gynecologic, or lung cancers with an expected prognosis of less than one year. The trial examines the effects of clinician- and patient-nudges on SIC documentation within 60 days of patient’s randomization to one of four arms (no nudge, clinician nudge, patient nudge, or both). This embedded mixed methods study recruited patients who still did not have an SIC 60 days from randomization, their caregivers (if present at time of consent) and oncologists. Interviews explored patients’ and caregivers’ 1) experiences with SICs, 2) perceptions about why SICs may not occur, and 3) suggestions to empower others to initiate SICs. Participating oncologists were surveyed by email. We used qualitative content analysis and descriptive statistics to identify themes. Results: The study included 44 participants: 19 patients, 10 caregivers and 15 oncologists. Patients had a median age of 63 years, were 63% female and 58% White and 68% had gastrointestinal cancers. Though no patient had a documented SIC, patients and their oncologists often disagreed about whether an undocumented SIC had occurred previously (Table). Patients’ and caregivers’ reported barriers to SICs included patient factors (lack of readiness, desire to maintain hope, focus on the present), clinician factors (perceived discomfort with prognostic disclosure and SICs), and prognostic uncertainty (current disease control, unpredictable prognosis). Patients’ and caregivers’ reported facilitators to SICs included patient- (self-advocacy, SIC readiness), clinician- (comfortable rapport), disease- (progression, worsening symptoms), and family-related facilitators (advocates for the patient to engage in SICs). To increase SICs, patients and caregivers reported greater acceptance of interventions involving a personal touch (e.g. phone call) over automated processes. Oncologist survey responses were brief. They reported SICs were most frequently triggered by disease progression or treatment intolerance, and recommended longer appointment times and written materials to facilitate SIC. Conclusions: Patient, caregiver and oncologist feedback on barriers and facilitators to SICs provided useful insights to improve interventions to encourage SICs. Patient/oncologist dyad (n=16) concordance of prior SICs. # of dyads Agreed SIC occurred 7 Agreed SIC did not occur 2 Only oncologist reported SIC 6 Only patient reported SIC 1 Total patient/oncologist dyads 16