[LarzI]

pawn Код:

if(!strcmp(cmdtext, "/b", true)) //Lenght is optional, so I'd rather not use it
{
    if(!cmdtext[2] || (cmdtext[2] == '\1' && !cmdtext[3])
        return SendClientMessage(playerid, COLOR_GREY, "USAGE: /b [OOC CHAT]");
    if(cmdtext[2] != '\1')
        return SendClientMessage(playerid, COLOR_GREY, "USAGE: /b [OOC CHAT] (don't forget to put a space between /b and your message)");
    new str[128];
    GetPlayerName(playerid,str, sizeof(str));
    format(str, sizeof(str), "%s ((%s))", str, cmdtext[3]);
    SendClientMessageToAll (COLOR_WHITE, str);
    return 1;
}

Problem solved

\1 = space ' '

pawn Код:

if(!cmdtext[2] || (cmdtext[2] == '\1' && !cmdtext[3])

Is doing the same as isnull does, aka recommended to use.