SIHOO M18 vs FlexiSpot OC3: Which Should You Buy?

Key difference: The OC3 has 2D armrests where the M18 has height-only arms, but the M18 is rated for more weight.

This page sets SIHOO's official figures for the M18 against FlexiSpot's for the OC3, with regular US list prices; it is not based on a Chairpedia test.

Why compare them

Why put the SIHOO M18 next to the FlexiSpot OC3? The SIHOO M18 and the FlexiSpot OC3 are both lower-priced mesh chairs sold through each brand's own store.

Specs side by side

Strengths and weaknesses

SIHOO M18

Strengths

• Higher maximum load: 330 lb (≈150 kg), against 300 lb (≈136 kg) on the OC3.
• Lumbar adjusts in two ways ("Lumbar support adjusts vertically and horizontally"), against one on the OC3.

Weaknesses

• Height-only armrests, against 2D on the OC3.
• Shorter warranty: 3 years, against 5 years on the OC3.
• Costs more: from $289.99 against $269.99 for the OC3.
• Shallower recline: up to 126° against 130° on the OC3.
• No seat-type choice is listed for the M18, unlike the OC3.

FlexiSpot OC3

Strengths

• 2D armrests, against height-only armrests on the M18.
• Longer warranty: 5 years, against 3 years on the M18.
• Lower list price: from $269.99 against $289.99 for the M18.
• Deeper recline: up to 130° against 126° on the M18.
• Choice of a foam or a mesh seat, which the M18's store listing does not offer.

Weaknesses

• Lower maximum load: 300 lb (≈136 kg), against 330 lb (≈150 kg) on the M18.
• Lumbar adjusts in one way only ("1.9" Height Adjustable"), against two on the M18.

How they differ

Armrests. The SIHOO M18 lists “Adjustable armrests (Up and down)”, the FlexiSpot OC3 “2D; 3.9" height adjustment”. With 2D armrests the OC3 lets you set the pads in more ways than the M18. The M18's pads only move up and down.

Warranty. The SIHOO M18 lists “3 years”, the FlexiSpot OC3 “5 years”. The OC3's cover runs longer.

Maximum load. The SIHOO M18 lists “330 lb (≈150 kg)”, the FlexiSpot OC3 “300 lb (≈136 kg)”. The M18 is rated for more weight.

Price. At regular US list prices the SIHOO M18 is $289.99–$299.99 and the FlexiSpot OC3 is $269.99. The OC3 lists $20 below the M18, before any store sale. Price options: the M18 — Black (incl. Skate Wheels) $289.99; White/Blue/Green/Pink $299.99; the OC3 — OC3, Black or Grey, foam or mesh seat (all four SKUs list at $269.99).

Lumbar support. The SIHOO M18 lists “Lumbar support adjusts vertically and horizontally”, the FlexiSpot OC3 “1.9" Height Adjustable”. The M18's lumbar adjusts in more ways than the OC3's.

Recline. The SIHOO M18 lists “Up to 126°”, the FlexiSpot OC3 “90°–130°”. The OC3 leans back further.

Seat options. The OC3 is sold with a foam or a mesh seat; the M18's store listing has no seat-type choice.

Seat size. Seat height: the SIHOO M18 lists nothing, the FlexiSpot OC3 “18.5–22.4 in (≈47–57 cm)”. Seat width: the SIHOO M18 lists nothing, the FlexiSpot OC3 “20.4 in (≈52 cm)”. Seat depth: the SIHOO M18 lists “17.91 in (≈45.5 cm)”, the FlexiSpot OC3 “15.9 in (≈40 cm)”.

Other details. Headrest: the SIHOO M18 lists “Headrest moves 3.9" up and down and pivots 45°”, the FlexiSpot OC3 “2D headrest; 2.1" height and 30° rotation”. Chair weight: the SIHOO M18 lists “37.1 lb (≈16.8 kg)”, the FlexiSpot OC3 “28.66 lb (≈13 kg)”.

Who should buy which

• Pick the M18 over the OC3 if you want a higher weight rating: 330 lb (≈150 kg).
• Pick the M18 over the OC3 if you want lumbar that adjusts in two ways rather than one.
• Pick the OC3 over the M18 if you want armrests that move in more directions than just up and down (2D).
• Pick the OC3 over the M18 if you want a longer warranty (5 years).
• Pick the OC3 over the M18 if you want the lower price — the OC3 lists $20 below the M18.

Not published by the brand (M18 vs OC3): seat height and seat width for the SIHOO M18.

Sources

• SIHOO M18 (compared here with the OC3): Sihoo M18 Ergonomic Office Chair – Official US Sihoo Store, checked 2026-09-29; US list price from SIHOO US Official Store, checked 2026-10-07.
• FlexiSpot OC3 (compared here with the M18): Essential Office chair (OC3) - FlexiSpot official store (SPEC tab), checked 2026-09-29; US list price from FlexiSpot US Store (flexispot.com), checked 2026-09-29.
• Figures marked ≈ for the M18 and OC3 are Chairpedia's unit conversions, not brand figures.