Ouist appears to simply be an alternate manner of spelling Uist, as evidenced by these references from 19th century books:
"On the 2nd, they arrived at the island of Erisca, belonging to Clan-ranald, which lies betwixt the isles of Barra and South Uist, or
Ouist..." [Forbes, R (1834)
Jacobite Memoirs of the Rebellion of 1745. Edinburgh: W & R Chambers.]
"When the herrings have once pushed their way to the southwards of the
Ouist Islands they make a shorter stay at Mull, Isla, Jura, and Argyleshire..." [Lifford, JH
et al (1861)
A collection of tracts and treatises... Dublin: A Thom.]
The fact that it doesn't match up with other stated birthplaces is another matter, but I think everyone would have at least one person in their tree who gives randomised birthplaces in different censuses. My g-g-g-grandfather was born in Woodlands, Dorset, or Bath, Somerset, depending on which census you look at.
All I'm saying is: don't discount it just yet
