I don't know for OCP as I've never used it.
But for standard screens, if you want to load them when space is pressed you want something like:
10 IF INKEY$=" " THEN LOAD"!screen.scr",&c000 ELSE GOTO 10
20 WHILE INKEY$<>" ":WEND
Replace the LOAD bit with the OCP specific load stuff which has been mentioned previously.
You can also use INKEY(47)>-1 and INKEY(47)=-1 in place of the respective INKEY$ instruction, either way works. This tests for the specific key (Space Bar = 47) being pressed rather than the space character being input. But unless you have changed the space bar with KEY DEF then it doesn't really matter.
Line 10 waits for space to be pressed, then loads the screen. Line 20 waits for space to be pressed before moving on/exiting.