The OMM Mountain Core 125 Sleeping Bag | Review - Outdoors Magic

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The OMM Mountain Core 125 Sleeping Bag | Review

An amazingly lightweight, Primaloft-insulated sleeping bag for mountain races and summer adventures

Why We Chose The OMM Mountain Core 125: Cosy interior, ultra-light design, super-compact, Primaloft Active

OMM (Original Mountain Marathon) are specialists in featherlight, minimalist gear designed for ultra marathons, hardcore trail running expeditions and ultra-light backpacking adventures. The OMM Mountain Core 125 sleeping bag is no exception. At 380g, it’s insanely light and packs down incredibly compactly (to a size slightly wider and taller than a 1L Jetboil pot). It’s far from the warmest sleeping bag on the market, and we certainly wouldn’t reach for it on anything other than the warmest of nights, but for balmy summer adventures where every gram counts this is a weight-saving dream.

Who Is The OMM Mountain Core 125 For?

This sleeping bag is perhaps best-suited to ultra-runners participating in long-distance, self-sufficient events involving overnight stops, where the lightest possible gear is required. It’ll also appeal to ultralight backpackers who are obsessed with getting their backpack weight as low as possible.

Photo: Chris Johnson

At 380g and sized 21x13cm when packed away, the minimalist approach is undoubtedly impressive – but this comes with a big compromise. The warmth provided from this sleeping is, unsurprisingly, minimal, so it’s only really suitable for warmer summer nights (or campers who don’t feel the cold too badly and are happy to prioritise weight over comfort). We imagine it’ll be used more abroad than in the UK, considering our relatively cold climate.

Warmth & Insulation

The insulation in this sleeping bag is Primaloft Active, which is used as the liner fabric to give an open, high-loft fleece feel. This delivers a good warmth-to-weight ratio. OMM state the Mountain Core 125 provides a 1.4 tog rating when dry, but no EN/ISO ratings (the usual comfort and comfort limit grades for sleeping bags) are published. One particular benefit of Primaloft Active is that it continues to insulate in wet conditions, and OMM claim a 1.0 tog rating if the sleeping bags gets wet from rain or condensation. There’s a 125g fill weight of Primaloft Active in this sleeping bag.

Photos: Chris Johnson


The fleecy feel of the Primaloft Active insulation in this bag is a big strength. The soft-textured plumes feel very comfy next to the skin, and deliver good breathability while also retaining heat. If you don’t like the clammy feel of other sleeping bags, this approach might prove a revelation. The sleeping environment is certainly pleasant and cosy.


As already discussed, the insulation is Primaloft Active, while the outer fabric is OMM’s in-house PointZero fabric – a lightweight fabric that’s highly windproof and has a DWR treatment for water resistance. The construction features a stitched-through inner while the outer is loose laid. Dimensions wise, the sleeping bag is 195cm long with a tapered mummy shape and a shoulder width of 65cm.


Due to the low weight, features are pretty minimalist, but you do get a half-length YKK zipper, a hood with an adjustable drawcord and hang loops at the base (for drying out the sleeping when required).

Tester’s Verdict

James Forrest, gear expert and outdoors writer

James keeping warm in the Mountain Core during our Test Team event in Snowdonia.

“For me, this sleeping bag has big pros and big cons. The negatives are that it’s so light and thinly insulated, I’d only ever really use it abroad in warmer climates, or on those rare occasions the UK is experiencing a heatwave. But I absolutely love how amazingly light and compact it is, and for fast and light backpacking trips in balmy climes it’s a really tempting, weight-saving choice. The interior is very comfy too, with the fleecy lining super soft and cosy against the skin – perfect for a good night’s sleep. If only it could magically be way warmer for the same weight.”

The OMM Mountain Core 125

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