When OpenSolaris boots it does not show kernel messages on the console by default. You can see them by passing an argument to kernel when booting. OpenSolaris uses grub boot loader. On the grub menu press "e" to enter edit mode.
Move to the line where kernel parameters are written with cursor keys.
Press "e" again for edit mode for the grub. Than add "-v" parameter to the end of kernel parameters.
After adding "-v" verbose parameter to kernel press "enter" and than "b" to boot OpenSolaris with verbose mode enabled.
If you need this option to be permanent, you need to modify grub config file in OpenSolaris. It's located in /boot/grub/menu.lst . If you use zfs root, than it's located in /rpool/boot/grub/menu.lst.You can do this on a Sun Sparc machine with Solaris when the openboot procedure begins by pressing "STOP-A" combination and type "boot -v"