To evaluate the safety and tolerability of DCR PHXC when administered monthly to patients with primary hyperoxaluria (PH) via change from baseline and incidence of abnormal physical exam findings.

A full physical examination will include a complete review of body systems: eyes, ears, nose, and throat, chest/respiratory, heart/cardiovascular, gastrointestinal/liver, musculoskeletal/extremities, dermatological/skin, thyroid/neck, lymph nodes, and neurological. A full physical exam is done at Screening, Day 180 and if a participant ends the study early.

A brief physical examination will minimally include chest/respiratory, heart/cardiovascular, dermatological/skin, and gastrointestinal/liver. A brief physical examination will be performed at the Investigator's discretion at all other visits.

To evaluate the safety and tolerability of DCR PHXC when administered monthly to patients with primary hyperoxaluria (PH) via the change from baseline and incidence of abnormal vital signs.

Vital signs include blood pressure, pulse/heart rate, oral body temperature, and respiratory rate.

Parameters will be measured in the supine position, using an automated instrument or manually, after the participant has rested comfortably for 10 minutes. In the pediatric population, an age-appropriate cuff size should be used for blood pressure measurements.

Temperature will be obtained in degrees Celsius (°C), pulse rate will be counted for a full minute and recorded in beats per minute, and respirations will be counted for a full minute and recorded in breaths per minute.

To evaluate the safety and tolerability of DCR PHXC when administered monthly to patients with primary hyperoxaluria (PH) via the change from baseline and incidence of abnormal clinical laboratory tests.

The proportion of participants with a 24 hour Uox level (< 0.46 mmol/24 hours or ≥ 0.46 - < 0.60 mmol/24 hours [adjusted per 1.73 m2 body surface area (BSA) in participants aged < 18 years]) at each assessment time point throughout the study in PH1, PH2, and PH3 participant subgroups

The percentage of participants with spot urinary oxalate-to-creatinine ratio ≤ the ULN or ≤ 1.5 x ULN at each assessment time point throughout the study in PH1, PH2, and PH3 participant subgroups

Change from Baseline in the number of stone events over a 12-month period, annually in Year 1, Year 2, etc. in PH1, PH2, and PH3 participant subgroups

Change from Baseline in the stone burden grade at Year 1, Year 2, etc. in PH1, PH2, and PH3 participant subgroups

Change from Baseline in nephrocalcinosis grade at Year 1, Year 2, etc. in PH1, PH2, and PH3 participant subgroups

The number of participants with severe CKD (GFR = 15-29 mL/min) or ESRD (GFR <15 mL/min); adjusted per 1.73 m2 BSA in participants aged < 18 years in PH1, PH2, and PH3 participant subgroups

To evaluate the effect of DCR-PHXC on Quality of Life (QoL) assessments in patients with PH.

The SF 36 is a set of generic, coherent, and easily administered quality-of-life measures that taps 8 health concepts: physical functioning, bodily pain, role limitations due to physical health problems, role limitations due to personal or emotional problems, emotional well-being, social functioning, energy/fatigue, and general health perceptions. It also includes a single item that provides an indication of perceived change in health. The 36 items are identical to the MOS SF 36 described in Ware and Sherbourne (1992). Participants respond to each item on a categorical scale. Categorical answers are transformed to a 0 to 100 range so that the lowest and highest possible scores are 0 and 100, respectively. All items are scored so that a high score defines a more favorable health state.

To evaluate the effect of DCR-PHXC on Quality of Life (QoL) assessments in patients with PH.

The EQ-5D-5L consists of the EQ 5D descriptive system and the EQ visual analogue scale (EQ VAS).

The descriptive system has 5 dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 5 levels: no problems, slight problems, moderate problems, severe problems and extreme problems. The digits for the 5 dimensions can be combined into a 5-digit number that describes the participant's health state.

The EQ VAS records the participant's self-rated health on a 20-cm vertical VAS, where the endpoints are labelled 'The best health you can imagine' and 'The worst health you can imagine.' Participants are asked to place an "X" on the line that represents their health on that day.

To evaluate the effect of DCR-PHXC on Quality of Life (QoL) assessments in patients with PH.

The 23-item PedsQL is comprised of 5 items in the Emotional, Social, and School Functioning dimensions (Psychosocial Health) and 8 items in the Physical Functioning (Physical Health) dimension. Items are reverse-scored on a 0 to 4 Likert scale and linearly transformed to a 0 to 100 scale, so that higher scores indicate better functioning and HRQOL. Scale Scores are the sum of the items in each dimension, divided by the number of items answered.

Time-weighted standardized area under the curve (TWS AUC) of 24-hour Uox from Day 90 to Day 180, based on percent change from Baseline in PH1, PH2, and PH3 participant subgroups. This endpoint will only be assessed in participants previously randomized to placebo in a previous study of DCR- PHXC and pediatric siblings.

Percent change from Baseline in 24-hour Uox at each assessment time point throughout the study in PH1, PH2, and PH3 participant subgroups. In those participants randomized to placebo in a previous study of DCR-PHXC and pediatric siblings, this endpoint will be assessed only after Month 6

Percent and absolute change from Baseline in spot urinary oxalate-to-creatinine ratio at each assessment time point throughout the study in PH1, PH2, and PH3 participant subgroups. In pediatric siblings, this endpoint will be assessed only after Month 6

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including time to maximum concentration (Tmax)

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including terminal elimination half-life (t1/2)

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including clearance (CL)

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including volume of distribution (V) estimates

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including secondary parameters of area under the curve (AUC)

Population and/or individual pharmacokinetic (PK) parameters for DCR PHXC, including maximum observed concentration (Cmax)