Is this the same as KSPO Dome?

No. KSPO Dome is a covered arena in Olympic Park; Olympic Stadium is the large outdoor bowl in Jamsil.

Are capacities fixed?

Published concert capacities change with stage and seating holds — read that date’s notice.

Venues · 잠실종합운동장 주경기장 — 위치·교통·스타디움 공연 패턴

蚕室オリンピックスタジアムとK-POP

Area: Jamsil, Songpa-gu — shared complex with indoor arena buildings; do not confuse the outdoor main stadium with Jamsil Indoor Stadium.
Transit: Fans typically use Jamsil station (Lines 2/8) and walk the complex paths; allow extra time for security lanes on stadium nights.
Entry: Expect larger perimeter checks, clear-bag or size limits when announced, and long walks between subway exits and gates.
Weather: Outdoor bowls mean rain plans and sun/heat prep matter more than at KSPO Dome.