From 2000-10-28 CYEG 'CYEG 281800Z AUTO 27004KT 1SM OBS000 02/02 A2952 RMK SLP021 T00240022 4////1064 51010 IEM_DS3505'

OBS is clearly "observation" but it's unclear what it means in this context.

Also seen in:
2001-06-27 CYEG
1981-08-15 CYOW
1984-10-13 CYOW (OBS004)
1988-07-05 CYGR (OBS014)
1984-10-20 CYYC (OBS001)
1992-10-18 (OBS002)
2001-08-10 CYWG
2001-11-02 CYSJ
2002-07-17 CYSJ