| Hardware and OS | vivo X21 UD | Oppo A3s |
| Operating system | Android 8.1 (Oreo); Funtouch 4 | Android 8.1 (Oreo); ColorOS 5.1 |
| Processor | Octa-core (4x2.2 GHz Kryo 260 & 4x1.8 GHz Kryo 260) | Octa-core 1.8 GHz Cortex-A53 |
| Chipset | Qualcomm SDM660 Snapdragon 660 (14 nm) | Qualcomm SDM450 Snapdragon 450 (14 nm) |
| GPU | Adreno 512 | Adreno 506 |
| Internal memory | 128 GB, 6 GB RAM | 16 GB, 2 GB RAM |
| External memory | microSD, up to 256 GB (uses SIM 2 slot) | microSD, up to 256 GB (dedicated slot) |
| Display | vivo X21 UD | Oppo A3s |
| Type | Super AMOLED | IPS LCD |
| Size | 6.28" inc | 6.2" inc |
| Resolution | 1080 x 2280 px | 720 x 1520 px |
| Touchscreen | capacitive touchscreen | capacitive touchscreen |
| Screen-to-Body Ratio | 85.2% | 81.2% |
| Display area | 98.4 cm2 | 95.9 cm2 |
| Display aspect ratio | 19:9 (height:width) | 19:9 (height:width) |
| Pixel Density | 402 PPI | 271 PPI |
| Colors | 16M | 16M |
| Connectivity | vivo X21 UD | Oppo A3s |
| 2G Network | GSM 850 / 900 / 1800 / 1900 - SIM 1 & SIM 2 | GSM 850 / 900 / 1800 / 1900 - SIM 1 & SIM 2 |
| 3G Network | HSDPA 850 / 900 / 1900 / 2100 | HSDPA 850 / 900 / 2100 |
| 4G Network | LTE band 1(2100), 2(1900), 3(1800), 5(850), 8(900), 34(2000), 38(2600), 39(1900), 40(2300), 41(2500) | LTE |
| Wi-Fi | Wi-Fi 802.11 a/b/g/n/ac, dual-band, WiFi Direct, hotspot | Wi-Fi 802.11 b/g/n, WiFi Direct, hotspot |
| Network | GSM / CDMA / HSPA / LTE | GSM / HSPA / LTE |
| SIM card | Hybrid Dual SIM (Nano-SIM, dual stand-by) | Dual SIM (Nano-SIM, dual stand-by) |
| Bluetooth | 5.0, A2DP, LE | 4.2, A2DP, LE |
| Edge | Yes | Yes |
| GPRS | Yes | Yes |
| GPS | Yes, with A-GPS, GLONASS | Yes, with A-GPS |
| Speed | HSPA 42.2/5.76 Mbps, LTE-A (2CA) Cat6 300/50 Mbps | HSPA 42.2/5.76 Mbps, LTE-A (2CA) Cat6 300/50 Mbps |
| USB | microUSB 2.0, USB On-The-Go | microUSB 2.0, USB On-The-Go |
| Camera | vivo X21 UD | Oppo A3s |
| Main camera | 12 MP, Dual | 13 MP, Dual |
| Specs | 12 MP, f/1.8, 1/2.55', 1.4µm, dual pixel PDAF
-5 MP, f/2.4, depth sensor | 13 MP, f/2.2, AF
-2 MP, f/2.4, depth sensor |
| Features | LED flash, HDR, panorama | LED flash, HDR, panorama |
| Video | 1080p@30fps | 1080p@30fps |
| Selfie camera | 12 MP, Single | 8 MP, Single |
| Multimedia | vivo X21 UD | Oppo A3s |
| 3.5 mm Jack | Yes | Yes |
| Radio | FM radio | FM radio |
| Design | vivo X21 UD | Oppo A3s |
| Colors | Black, Ruby Red | Red, Dark Purple |
| Dimensions | 154.5 x 74.8 x 7.4 mm | 156.2 x 75.6 x 8.2 mm |
| Weight | 156.2 g | 168 g |
| Body build | Front glass, aluminum body | |
| Battery | vivo X21 UD | Oppo A3s |
| Battery capacity | 3200 mAh, Non-removable Li-Ion | 4230 mAh, Non-removable Li-Ion |
| Other | vivo X21 UD | Oppo A3s |
| Release date | 2018, March | 2018, July |
| Sensors | Fingerprint (under display), accelerometer, proximity, compass | Accelerometer, proximity, compass |
