Kelly Osbourne is putting any worries about her recent weight loss to bed ... stepping out in London looking confident, glamorous, and in great spirits.

Check out the pics ... Kelly hit up the Vivienne Westwood x J. Sheekey launch Monday night in London, rocking a sleek black dress, sparkling jewels and a full glam look that had her shining from every angle.

Kelly was all smiles as she posed for photos, mingled with guests, and soaked up the atmosphere ... looking far removed from the difficult months she's endured.

As you know, Kelly's had a rough stretch after losing her dad -- Ozzy Osbourne -- last year. Sources previously told us her weight loss was largely tied to the grief she experienced following her father's death.

Kelly's slimmer frame has sparked plenty of chatter online, with some trolls taking aim at her appearance ... and she hasn't been shy about firing back at her critics.