The pair have put pen to paper on three-year deals only days after the Ospreys snapped up All Black star Justin Marshall and hugely promising Wales Under-21 fly-half James Hook.
Former All Black No 8 Filo Tiatia is another high-profile signing, and lock Jono Gibbes - who led New Zealand Maori to their win over the British Lions last year - is also close to joining the west Wales region.
Ospreys coach Lyn Jones said: "Lee has developed this season into a fully fledged international calibre player.
"We are delighted that he has chosen to get on board with the Ospreys. He played a prominent role in the Six Nations with Wales and is a full-back of some considerable stature who will fit in well at the region."