densification then: 198 O'Connor

 The Kitchener Apartments at 198 O'Connor were built in 1914. The honorific likely refers to the British War Secretary Herbert Kitchener, who was at the time assembling a large volunteer army in the early days of WWI.

WWI-era patriotic postcard

The Kitchener replaced a Victorian 2½-storey house of a build similar to the two we see flanking it to this day, explaining why it looks to have been somewhat shoehorned into the lot. 

The original #198 — Goad, 1912

Advertising for the Kitchener began in late 1914 — "up-to-date, latest improvements" — and the 1915 Might directory lists twelve occupants including a doctor's office on the ground floor.

Ottawa Journal — December 7 1914

Might Directory for Ottawa — 1915