Online Tournaments Rating Systems is completely based on the following two simple formulas:

1) Performance Calculation

R(P) = R(avv) + 400 * ( W - L ) / N

where:

R(avv) is the mean value of opponents ratings;
N  is the number of played games;
W is the number of won games;
L  is the number of lost games;

2) Rating Variation;

R(N) = R(O) + 16 * ( W - L ) + 0.04 * D

where:

R(N) is the rating after a tourney;
R(O) is the rating before a tourney
W      is the number of won games;
L       is the number of lost games;
D      is the sum of rating differences between each opponent and
          player's rating, and each difference is truncated at
          350 or -350.

Previous formulas, roughly correspond to FIDE tables, with K=32 .

Each new player, receives a provisional rating of 1600 .
After 20 games (more or less, see below), he receives a final rating, calculated from performances formula (1).
Performances formula is applied to all the games of tourneys including the one in which the player playes his 20th games. All games played in that tourney, after the 20th, are included in the calculation.
When a player receives a final rating, this starts varying according formula (2)
Rating variations are calculated after each and every tourney, and starting rating is the rating at the beginning of the tourney.
Games won/lost by forfeit, are counted in OTR variations.
OTR list is available here.