I believe it is hrSystemUptime from the Host Resources mib:
hrSystemUptime OBJECT-TYPE
SYNTAX TimeTicks (0 .. 4294967295)
ACCESS read-only
STATUS current
DESCRIPTION
"The amount of time since this host was last
initialized. Note that this is different from
sysUpTime in the SNMPv2-MIB [RFC1907] because
sysUpTime is the uptime of the network management
portion of the system."
::= { hrSystem 1 }