In article , "Phillip Helbig (undress to
reply)" writes:

In one notation (unfortunately, there are several)

Omega_M + Omega_Lambda + Omega_K = 1

or

Omega + lambda + Omega_L = 1

Above line should be

Omega + lambda + Omega_K = 1

Hence Omega + lambda = 1 if the universe is flat.

Yes, different notation schemes can be confusing, on the other hand
understanding them can help one learn other things.

In the scheme above,

Omega_K := 1 - Omega - lambda

or

Omega_K := 1 - Omega_M - Omega_lambda

One can also find

Omega_K := Omega_M + Omega_lambda - 1

or

Omega_K := Omega + lambda - 1

i.e. the sign of Omega_K is opposite.

I prefer the latter, since then Omega_K (or K, in some notation) has the
same sign as k, the curvature constant in the Friedmann equation.