The only-loadable option lets you specify that a module can only be brought into virtual storage using a LOAD macro instruction.

A module with the only-loadable attribute must be entered with a branch instruction or a CALL macro instruction. If an attempt is made to enter the module with a LINK, XCTL, or ATTACH macro instruction, the program making the attempt is terminated abnormally by the control program. (See z/OS MVS Programming: Assembler Services Guide for information on the LINK, XCTL, and ATTACH macro instructions.)

OL=NO is the default value and can also be specified with the keyword NOOL.